Semiconductor device and electronic device

ABSTRACT

A semiconductor device in which a circuit and a power storage element are efficiently placed is provided. The semiconductor device includes a first transistor, a second transistor, and an electric double-layer capacitor. The first transistor, the second transistor, and the electric double-layer capacitor are provided over one substrate. A band gap of a semiconductor constituting a channel region of the second transistor is wider than a band gap of a semiconductor constituting a channel region of the first transistor. The electric double-layer capacitor includes a solid electrolyte.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice and an electronic device.

Further, one embodiment of the present invention relates to an object, amethod, or a manufacturing method. Further, the present inventionrelates to a process, a machine, manufacture, or a composition ofmatter. In addition, one embodiment of the present invention relates toa semiconductor device, a display device, a light-emitting device, apower storage device, a memory device, a driving method thereof, or amanufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A display device, an electro-optical device, asemiconductor circuit, and an electronic device include a semiconductordevice in some cases.

2. Description of the Related Art

Electronic devices carried around by the users and electronic devicesworn by the users have been actively developed in recent years.

Since electronic devices carried around by the users and electronicdevices worn by the users are powered by power storage devices, theirpower consumption is reduced as much as possible. Particularly in thecase where a central processing unit (CPU), which consumes a lot ofpower for its operation, is included in the electronic device,processing of the CPU greatly affects the power consumption of theelectronic device.

A semiconductor device including a high-performance integrated circuit(e.g., a CPU) on a plastic or plastic film substrate, which transmitsand receives electric power or signals wirelessly, is described inPatent Document 1.

A semiconductor device in which a register in a CPU is formed using amemory circuit including an oxide semiconductor transistor to reducepower consumption is described in Patent Document 2.

A technique for fabricating an electric double-layer capacitor (EDLC)including a solid electrolyte is proposed in Patent Document 3.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No. 2006-32927

[Patent Document 2] Japanese Published Patent Application No.2013-251884

[Patent Document 3] Japanese Published Patent Application No.2013-191769

SUMMARY OF THE INVENTION

The detail of the power consumption of an electronic device including aCPU will be described. The power consumption can be roughly classifiedinto power consumed by a CPU, power consumed by systems around the CPU,and power consumed by a plurality of input/output devices and peripheraldevices connected to the inside or outside of the electronic device. Thepower consumed by systems around the CPU includes a loss in a converter,a loss in a wiring pattern, and power consumed by a bus, a controller,and the like.

When an electronic device is reduced in size or thickness, a powerstorage element such as a battery or an EDLC is also subjected to thelimitation. However, as the power storage element decreases in area, thecapacitor thereof also decreases. Thus, a circuit, a power storageelement, and the like need to be provided in a smaller space.

Furthermore, a power storage element generates heat by charge anddischarge, which may thermally affect the surrounding area.

As an electronic device is reduced in size and a circuit, a powerstorage element, and the like are stored in a smaller space, control ofthe power consumption and heat generation becomes a problem.

One embodiment of the present invention provides a novel semiconductordevice, a semiconductor device in which a circuit and a power storageelement are efficiently stored, a semiconductor device with small powerconsumption, or a semiconductor device with reduced heat generation.

One embodiment of the present invention provides an electronic devicehaving a novel structure. Specifically, an electronic device having anovel structure that can have various forms is provided. Morespecifically, a wearable electronic device that is used while being wornon the body and an electronic device that is used while being implantedin the body are provided.

Note that the description of a plurality of objects does not mutuallypreclude their existence. Note that one embodiment of the presentinvention does not necessarily achieve all the objects listed above.Objects other than those listed above are apparent from the descriptionof the specification, drawings, and claims, and such objects could be anobject of one embodiment of the present invention.

One embodiment of the present invention is a semiconductor device thatincludes a first transistor, a second transistor, and an electricdouble-layer capacitor. The first transistor, the second transistor, andthe electric double-layer capacitor are preferably provided over onesubstrate. The first transistor includes a first semiconductor in achannel region. The second transistor includes a second semiconductor ina channel region. A band gap of the second semiconductor is preferablywider than a band gap of the first semiconductor. The electricdouble-layer capacitor preferably includes a solid electrolyte.

One embodiment of the present invention is a semiconductor device thatincludes a first transistor, a second transistor, and an electricdouble-layer capacitor. The first transistor, the second transistor, andthe electric double-layer capacitor are preferably provided over onesubstrate. The first transistor includes a first semiconductor in achannel region. The second transistor includes a second semiconductor ina channel region. A band gap of the second semiconductor is preferablywider than a band gap of the first semiconductor. The electricdouble-layer capacitor preferably includes an ionic liquid.

In the above embodiments, the first semiconductor preferably includessilicon and the second semiconductor preferably includes an oxidesemiconductor.

In the above embodiments, the electric double-layer capacitor ispreferably capable of being charged wirelessly.

In the above embodiments, a semiconductor substrate may be used as thesubstrate.

In the above embodiments, a flexible substrate may be used as thesubstrate.

One embodiment of the present invention is an electronic device thatincludes the semiconductor device according to any of the aboveembodiments and at least one of a microphone, a speaker, a displayportion, and an operation key.

Note that the power storage element in this specification and the likerefers to all elements that have the function of storing electric power.For example, a lithium-ion secondary battery, a lithium-ion capacitor,and an electric double-layer capacitor are included in the category ofthe power storage element.

Furthermore, the power storage device in this specification and the likerefers to all devices that include the power storage elements.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over” “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Further, the positional relation between components is changedas appropriate in accordance with a direction in which each component isdescribed. Thus, the positional relation is not limited to thatdescribed with a term used in this specification, and can be explainedwith another term as appropriate in accordance with the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode layer) and a source (a sourceterminal, a source region, or a source electrode layer), and current canflow through the drain, the channel region, and the source. Note that inthis specification and the like, a channel region refers to a regionthrough which current mainly flows.

Further, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that the terms “film” and “layer” can be interchanged with eachother in accordance with the case or circumstances. For example, theterm “conductive layer” can be changed into the term “conductive film”in some cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

According to one embodiment of the present invention, it becomespossible to provide a novel semiconductor device, a semiconductor devicein which a circuit and a power storage element are efficiently stored, asemiconductor device with small power consumption, or a semiconductordevice with reduced heat generation.

Furthermore, according to one embodiment of the present invention, itbecomes possible to provide an electronic device having a novelstructure. Specifically, it becomes possible to provide an electronicdevice having a novel structure that can have various forms. Morespecifically, it becomes possible to provide a wearable electronicdevice that is used while being worn on the body and an electronicdevice that is used while being implanted in the body.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram showing a structural example of asemiconductor device;

FIG. 2 is a cross-sectional view showing a structural example of asemiconductor device;

FIG. 3 is a cross-sectional view of a transistor;

FIG. 4 is a cross-sectional view showing a structural example of asemiconductor device;

FIG. 5A is a top view and FIGS. 5B and 5C are cross-sectional viewsshowing structural examples of an electric double-layer capacitor;

FIG. 6A is a top view and FIGS. 6B and 6C are cross-sectional viewsshowing structural examples of an electric double-layer capacitor;

FIG. 7A is a top view and FIG. 7B is a cross-sectional view showing astructural example of an electric double-layer capacitor;

FIGS. 8A to 8D are cross-sectional views illustrating a manufacturingmethod of an electric double-layer capacitor;

FIG. 9A is a top view and FIG. 9B is a cross-sectional view showing astructural example of an electric double-layer capacitor;

FIGS. 10A and 10B are cross-sectional views showing structural examplesof an electric double-layer capacitor;

FIG. 11A is a top view and FIGS. 11B to 11D are cross-sectional viewsshowing structural examples of a transistor;

FIG. 12A is a cross-sectional view showing a structural example of atransistor and FIG. 12B is an energy band diagram of the transistor;

FIG. 13 is a cross-sectional view showing a structural example of atransistor;

FIG. 14 is a cross-sectional view showing a structural example of atransistor;

FIG. 15 is a cross-sectional view showing a structural example of atransistor;

FIGS. 16A to 16F are perspective views each showing an example of anelectronic device;

FIGS. 17A and 17B each illustrate an example of an electronic device;

FIG. 18 illustrates an example of an electronic device;

FIGS. 19A and 19B each illustrate an example of an electronic device;

FIGS. 20A and 20C are top views and FIG. 20B is a perspective viewshowing examples of an electronic device;

FIG. 21 is a cross-sectional view showing a structural example of asemiconductor device; and

FIG. 22A is a top view and FIG. 22B is a cross-sectional view showing astructural example of a transistor.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented with various modes. It willbe readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments. In addition, in the following embodiments and examples, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and description thereofwill not be repeated.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to the shapes or values shown inthe drawings.

Embodiment 1

<Block Diagram of Device>

FIG. 1 is a block diagram of a device 10 that is one embodiment of thepresent invention.

The device 10 of this embodiment includes a control module 15, a displaymodule 21, and a communication module 26. The control module 15 is acontroller that controls the entire device 10, communication, anddisplay of information on a display portion 16.

The control module 15 includes a CPU 11, a power storage element 12, aregulator 13, and a wireless receiving portion 14.

The display module 21 includes the display portion 16, a display drivercircuit 19, a power storage element 17, a regulator 18, and a wirelessreceiving portion 20.

The communication module 26 includes a communication circuit 22, a powerstorage element 23, a regulator 24, and a wireless receiving portion 25.

A regulator is an electronic circuit that keeps an output voltage orcurrent constant. A regulator is classified into two kinds, a linearregulator and a switching regulator, that can be used in accordance withthe amount of electric load or the like. A switching regulator is alsocalled a DC-DC converter.

Each module includes a regulator and a power storage element. Each powerstorage element can regain its continuous operating time by means ofcharging, and is connected to a circuit so that it can be wirelesslycharged. The power storage elements are electrically connected to therespective wireless receiving portions via the respective regulators.Each regulator supplies necessary electric power or signals to thefunctional circuit, from the connected power storage element. Inaddition, each regulator also has a function of preventing overchargeand the like when the connected power storage element is charged.

It is preferable that the power storage element in each module beprovided over the same substrate as the functional circuits included inthe same module. In this manner, the device 10 can be reduced in size orthickness.

In the device 10, each of the modules can be turned on or turned offindependently. The operating system that selectively drives only themodule to be used can reduce power consumption of the device 10.

For example, when the user looks at information on the display portion16 without using a communication function, the communication circuit 22is in an off state where the power storage element 23 is not used inorder that electric power to the communication circuit 22 is blocked inthe communication module 26, while the display module 21 and the controlmodule 15 are in an on state.

Furthermore, for a still image, once the still image is displayed on thedisplay portion 16 with the display module 21 and the control module 15being in an on state, the still image can be kept displayed while onlythe display module 21 is in an on state even after the control module 15is turned off with the still image being displayed. In that case, thecontrol module 15 is not operated although the still image is displayed,and the power consumed by the control module 15 can apparently be zero.Note that when transistors of the display portion 16 include an oxidesemiconductor with low off-state current (e.g., an oxide materialincluding In, Ga, and Zn), or when the display portion 16 includes amemory for each of the pixels, the still image can be kept displayed fora certain period even when the supply of electric power from the powerstorage element 17 is blocked after the still image is displayed.

In this manner, a power storage element is provided for each of thecomponents to be used in the electronic device, and only the componentin use is selectively driven, whereby the power consumption can bereduced.

It is preferable that an EDLC be used for the power storage element ineach module. An EDLC is capable of being charged and discharged at highspeed although it has large capacity. Thus, the device 10 can be drivenat high speed.

The electrolyte of the EDLC is preferably solid. A solid electrolyte issafe because it has no risk of spill as compared to a liquid electrolyteand it can be used at a higher temperature than room temperature.

Further, the EDLC preferably includes an ionic liquid that hasnon-flammability and non-volatility as the electrolyte. The use of anionic liquid can prevent a power storage element from exploding orcatching fire even when the EDLC internally shorts out or the internaltemperature increases owing to overcharging or the like.

Note that a memory cell including an oxide semiconductor transistor ispreferably used for a register in the CPU 11. With the use of an oxidesemiconductor transistor in the CPU 11, even in the case where theoperation of the CPU 11 is temporarily stopped and the supply of thepower supply voltage is stopped, data can be held and power consumptioncan be reduced. Specifically, for example, while a user of a personalcomputer does not input data to an input device such as a keyboard, theoperation of the CPU 11 can be stopped, whereby the power consumptioncan be reduced.

Furthermore, the use of oxide semiconductor transistors as thetransistors used for the regulators 13, 18, and 24 can reduce powerconsumption because of the small off-state current. In particular, aregulator (DC-DC converter) including a control circuit including oxidesemiconductor transistors can operate at a temperature of 150° C. orhigher. Thus, such a DC-DC converter of an embodiment is preferably usedfor an electronic device that is likely to operate at high temperatures.

In this embodiment, an example in which the display module 21, thecontrol module 15, and the communication module 26 each have a powerstorage element is described; however, the total number of power storageelements is not limited to three. The electronic device may additionallyinclude functional modules and their power storage elements, in whichcase the electronic device has four or more power storage elements intotal.

For example, if an exterior body of the device 10 is formed of aflexible material, a wearable device that is used while being worn onthe body can be provided. In that case, if small-sized power storageelements are dispersedly arranged in the device 10, a feeling of weightcan be reduced as compared to an electronic device having a single largepower storage element. In addition, even if the individual small-sizedpower storage element generates heat, it does not ruin the comfort whenwearing the device.

Next, a semiconductor device that can be used for the device 10 will bedescribed with reference to FIGS. 2 to 4.

Structural Example 1 of Semiconductor Device

FIG. 2 shows a cross-sectional view of a semiconductor device 1000including a first transistor 720, a second transistor 730, and a powerstorage element 740 that are formed over the same substrate. The firsttransistor 720 is provided over a substrate 700, the second transistor730 is provided over the first transistor 720, and the power storageelement 740 is provided over the second transistor 730.

A channel region of the second transistor 730 preferably includes asemiconductor different from that of a channel region of the firsttransistor 720. Specifically, the second transistor 730 preferablyincludes a semiconductor with a wider band gap (wide band gapsemiconductor) than that of a semiconductor in the first transistor 720.For example, it is preferable that the semiconductor material in thefirst transistor 720 be silicon, germanium, silicon germanium, siliconcarbide, gallium arsenide, or the like, and that the semiconductormaterial in the second transistor 730 be an oxide semiconductor. Atransistor using single crystal silicon or the like as a semiconductormaterial can easily operate at high speed. In contrast, a transistorincluding an oxide semiconductor has a low off-state current.

The power storage element 740 corresponds to the power storage elementincluded in each of the modules of the device 10. A power storageelement that can regain its continuous operating time by means ofcharging is preferably used as the power storage element 740. Inparticular, an EDLC is preferable. An EDLC is capable of being chargedand discharged at high speed although it has large capacity. Thus, thesemiconductor device 1000 can be driven at high speed.

The electrolyte of the EDLC is preferably solid. A solid electrolyte issafe because it has no risk of spill as compared to a liquid electrolyteand it can be used at a higher temperature than room temperature.

Further, the EDLC preferably includes an ionic liquid that hasnon-flammability and non-volatility as the electrolyte. The use of anionic liquid can prevent a power storage element from exploding orcatching fire even when the EDLC internally shorts out or the internaltemperature increases owing to overcharging or the like.

The semiconductor device 1000 includes the substrate 700, the firsttransistor 720, an element isolation layer 727, an insulating film 731,the second transistor 730, an insulating film 732, an insulating film741, the power storage element 740, an insulating film 742, plugs 701,702, 703, and 704, and wirings 705, 706, 707, and 708. The firsttransistor 720 includes a gate electrode 726, a gate insulating film724, a sidewall insulating layer 725, an impurity region 721 functioningas a source region or a drain region, an impurity region 722 functioningas a lightly doped drain (LDD) region or an extension region, and achannel region 723.

The impurity concentration is higher in the impurity region 721 than inthe impurity region 722. The impurity region 721 and the impurity region722 can be formed in a self-aligned manner, with the gate electrode 726and the sidewall insulating layer 725 used as a mask.

As the substrate 700, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate of silicon or silicon carbide, acompound semiconductor substrate of silicon germanium, asilicon-on-insulator (SOI) substrate, or the like can be used. Atransistor manufactured using a semiconductor substrate can operate athigh speed easily. In the case where a p-type single crystal siliconsubstrate is used as the substrate 700, an impurity element impartingn-type conductivity may be added to part of the substrate 700 to form ann-well, and a p-type transistor can be formed in a region where then-well is formed. As the impurity element imparting n-type conductivity,phosphorus (P), arsenic (As), or the like can be used. As the impurityelement imparting p-type conductivity, boron (B) or the like may beused.

Alternatively, the substrate 700 may be a metal substrate or insulatingsubstrate over which a semiconductor film is provided. Examples of themetal substrate are a stainless steel substrate, a substrate includingstainless steel foil, a tungsten substrate, and a substrate includingtungsten foil. Examples of the insulating substrate are a glasssubstrate, a quartz substrate, a plastic substrate, a flexiblesubstrate, an attachment film, paper including a fibrous material, and abase film. As an example of a glass substrate, a barium borosilicateglass substrate, an aluminoborosilicate glass substrate, a soda limeglass substrate, or the like can be given. Examples of a flexiblesubstrate include a flexible synthetic resin such as plastics typifiedby polyethylene terephthalate (PET), polyethylene naphthalate (PEN), andpolyether sulfone (PES), and acrylic. Examples of an attachment film areattachment films formed using polypropylene, polyester, polyvinylfluoride, polyvinyl chloride, and the like. Examples of a base film arebase films formed using polyester, polyamide, polyimide, aramid, epoxy,an inorganic vapor deposition film, and paper.

Alternatively, a semiconductor element may be formed using onesubstrate, and then, transferred to another substrate. Examples of asubstrate to which a semiconductor element is transferred include, inaddition to the above-described substrates, a paper substrate, acellophane substrate, an aramid film substrate, a polyimide filmsubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester, or the like), aleather substrate, and a rubber substrate. When such a substrate isused, a transistor with excellent properties or a transistor with lowpower consumption can be formed, a device with high durability, highheat resistance can be provided, or reduction in weight or thickness canbe achieved.

The first transistor 720 is isolated from other transistors formed onthe substrate 700 by an element isolation layer 727.

As the first transistor 720, a transistor containing silicide (salicide)or a transistor which does not include a sidewall insulating layer maybe used. When a structure that contains silicide (salicide) is used,resistance of the source region and the drain region can be furtherlowered and the speed of the semiconductor device can be increased.Further, the semiconductor device can be operated at low voltage; thus,power consumption of the semiconductor device can be reduced.

The second transistor 730 is an oxide semiconductor transistor. Thedetails of the second transistor 730 will be described later inEmbodiment 3.

Here, in the case where a silicon-based semiconductor material is usedfor the first transistor 720 provided in a lower portion, hydrogen in aninsulating film provided in the vicinity of the semiconductor film ofthe first transistor 720 terminates dangling bonds of silicon;accordingly, the reliability of the first transistor 720 can beimproved. Meanwhile, in the case where an oxide semiconductor is usedfor the second transistor 730 provided in an upper portion, hydrogen inan insulating film provided in the vicinity of the semiconductor film ofthe second transistor 730 becomes a factor of generating carriers in theoxide semiconductor; thus, the reliability of the second transistor 730might be decreased. Therefore, in the case where the second transistor730 using an oxide semiconductor is provided over the first transistor720 using a silicon-based semiconductor material, it is particularlyeffective that the insulating film 731 having a function of preventingdiffusion of hydrogen is provided between the first transistor 720 andthe second transistor 730. The insulating film 731 makes hydrogen remainin the lower portion, thereby improving the reliability of the firsttransistor 720. In addition, since the insulating film 731 suppressesdiffusion of hydrogen from the lower portion to the upper portion, thereliability of the second transistor 730 can also be improved.

The insulating film 731 can be, for example, formed using aluminumoxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttriumoxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ).

Furthermore, the insulating film 732 having a function of preventingdiffusion of hydrogen is preferably formed over the second transistor730 to cover the second transistor 730 including an oxide semiconductorfilm. For the insulating film 732, a material that is similar to that ofthe insulating film 731 can be used, and in particular, an aluminumoxide film is preferably used. An aluminum oxide film has a highshielding (blocking) effect of preventing penetration of both oxygen andimpurities such as hydrogen and moisture. Thus, by using an aluminumoxide film as the insulating film 732 covering the second transistor730, release of oxygen from the oxide semiconductor film included in thesecond transistor 730 can be prevented and entry of water and hydrogeninto the oxide semiconductor film can be prevented.

The plugs 701 to 704 and the wirings 705 to 707 preferably have asingle-layer structure or a stacked-layer structure of a conductive filmcontaining a low-resistance material selected from copper (Cu), tungsten(W), molybdenum (Mo), gold (Au), aluminum (Al), manganese (Mn), titanium(Ti), tantalum (Ta), nickel (Ni), chromium (Cr), lead (Pb), tin (Sn),iron (Fe), and cobalt (Co), an alloy of such a low-resistance material,or a compound containing such a material as its main component. It isparticularly preferable to use a high-melting-point material that hasboth heat resistance and conductivity, such as tungsten or molybdenum.In addition, the plugs and wirings are preferably formed using alow-resistance conductive material such as aluminum or copper. Further,the plugs and wirings are preferably formed using a Cu—Mn alloy, sincein that case, manganese oxide formed at the interface with an insulatorcontaining oxygen has a function of preventing Cu diffusion.

The power storage element 740 is an electric double-layer capacitor thatcan regain its continuous operating time by means of charging, and anall-solid-state battery including a solid electrolyte. Furthermore, inorder to enable wireless charging, the power storage element 740 iselectrically connected to a wireless receiving portion via a regulator.

The power storage element 740 may be fabricated through a semiconductormanufacturing process. Note that the semiconductor manufacturing processrefers to methods in general that are used for manufacturingsemiconductor devices, such as a film formation process, acrystallization process, a plating process, a cleaning process, alithography process, an etching process, a polishing process, animpurity implantation process, or a heat treatment process.

The details of the power storage element 740 will be described later inEmbodiment 2.

The insulating film 741 can be formed to have a single-layer structureor a stacked-layer structure using one or more of silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum nitride, aluminum oxynitride, hafnium oxide, zirconium oxide,yttrium oxide, gallium oxide, lanthanum oxide, cesium oxide, tantalumoxide, and magnesium oxide.

In the case where the power storage element 740 includes lithium, theinsulating film 741 preferably has a function of preventing (blocking)diffusion of lithium. When lithium that is included in the power storageelement 740 enters a semiconductor element (the first transistor 720 orthe second transistor 730) as a movable ion, the semiconductor elementdeteriorates. With the insulating film 741 blocking lithium ions, ahighly reliable semiconductor device can be provided.

In the case where the power storage element 740 includes lithium, theinsulating film 741 preferably includes halogen such as fluorine,chlorine, bromine, or iodine. When the insulating film 741 includeshalogen, the halogen is easily combined with lithium that is an alkalimetal. Then, lithium is fixed in the insulating film 741, wherebydiffusion of lithium to the outside of the insulating film 741 can beprevented.

In the case where the insulating film 741 is formed of silicon nitrideby a chemical vapor deposition (CVD) method, for example, when ahalogen-containing gas is mixed in a source gas at 3% to 6% (volumeratio), e.g., at 5%, the obtained silicon nitride film includes thehalogen. The concentration of the halogen element included in theinsulating film 741, measured by secondary ion mass spectrometry (SIMS),is greater than or equal to 1×10¹⁷ atoms/cm³, preferably greater than orequal to 1×10¹⁸ atoms/cm³, and more preferably greater than or equal to1×10¹⁹ atoms/cm³.

The insulating film 742 has a function of protecting the power storageelement 740. As the insulating film 742, for example, an insulatingmaterial such as a resin (e.g., a polyimide resin, a polyamide resin, anacrylic resin, a siloxane resin, an epoxy resin, or a phenol resin),glass, an amorphous compound, or ceramics can be used. Furthermore, alayer containing calcium fluoride or the like may be provided as a waterabsorption layer between resin layers. The insulating film 742 can beformed by a spin coating method, an ink jet method, or the like.Alternatively, the insulating film 742 can be formed to have asingle-layer structure or a stacked-layer structure using one or more ofsilicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, aluminum nitride, aluminum oxynitride, hafniumoxide, zirconium oxide, yttrium oxide, gallium oxide, lanthanum oxide,lanthanum oxide, cesium oxide, tantalum oxide, and magnesium oxide.

The semiconductor device 1000 may further include a semiconductorelement over the power storage element 740. In that case, the insulatingfilm 742 preferably has a function of preventing (blocking) diffusion oflithium, as with the insulating film 741. With the insulating film 742blocking lithium, a highly reliable semiconductor device can beprovided.

In the case where a semiconductor element is formed over the powerstorage element 740, the insulating film 742 preferably includes halogensuch as fluorine, chlorine, bromine, or iodine, as with the insulatingfilm 741. With the insulating film 742 including halogen, the halogen iseasily combined with lithium that is an alkali metal, whereby diffusionof lithium to the outside of the insulating film 742 can be prevented.

In FIGS. 2 to 4, regions where reference numerals and hatching patternsare not given show regions formed of an insulator. The region can beformed using an insulator containing at least one of aluminum oxide,aluminum nitride oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, tantalum oxide, and the like.Alternatively, in these regions, an organic resin such as a polyimideresin, a polyamide resin, an acrylic resin, a siloxane resin, an epoxyresin, or a phenol resin can be used.

The semiconductor device 1000 in FIG. 2 preferably includes a coolingdevice such as a heat sink, a water-cooling cooler, or a cooling fanover the power storage element 740. The provision of the cooling devicecan prevent a malfunction of the semiconductor device 1000 caused byheat generation of the power storage element 740.

The semiconductor device 1000 in FIG. 2 may include an air gap (a spaceof a vacuum layer) between the power storage element 740 and the firstand second transistors 720 and 730. The provision of the air gap canprevent heat generated in the power storage element 740 from beingconducted to the first transistor 720 and the second transistor 730.Thus, malfunctions of the first transistor 720 and the second transistor730 caused by heat can be prevented.

Although the power storage element 740 is provided over the firsttransistor 720 and the second transistor 730 in FIG. 2, the powerstorage element 740 may be provided between the first transistor 720 andthe second transistor 730. In that case, the first transistor 720, thepower storage element 740, and the second transistor 730 are formed inthis order. In the case where formation of the power storage element 740requires heat treatment at such a high temperature as will destroy thesecond transistor 730 in particular, it is preferable to form the secondtransistor 730 after forming the power storage element 740.

The circuits of the CPU 11 or the regulator 13 and the like included inthe device 10 in FIG. 1 are fabricated using the first transistor 720and the second transistor 730, and the power storage element 740 isfabricated over the same substrate as that of the circuits, for example,whereby the device 10 can be reduced in size or thickness.

Structural Example 2 of Semiconductor Device

Although the first transistor 720 in FIG. 2 is a planar transistor, theform of the first transistor 720 is not limited thereto. For example, aFIN-type or TRI-GATE transistor 750 shown in FIG. 3 may be used as thefirst transistor 720.

FIG. 3 shows cross-sectional views of the transistor 750. The leftcross-sectional view shows a cross section of the transistor 750 in thechannel length direction, and the right cross-sectional view shows thatin the channel width direction.

In FIG. 3, an insulating film 757 is provided over the substrate 700.The substrate 700 includes a protruding portion with a thin tip (alsoreferred to as a fin).

Note that an insulating film may be provided over the protrudingportion. The insulating film functions as a mask for preventing thesubstrate 700 from being etched when the projecting portion is formed.Alternatively, the protruding portion may not have the thin tip; aprotruding portion with a cuboid-like protruding portion and aprotruding portion with a thick tip are permitted, for example. A gateinsulating film 754 is provided over the protruding portion of thesubstrate 700, and a gate electrode 756 and a sidewall insulating layer755 are formed thereover. In the substrate 700, an impurity region 751functioning as a source region or a drain region, an impurity region 752functioning as an LDD region or an extension region, and a channelregion 753 are formed. Note that here is shown an example in which thesubstrate 700 includes the protruding portion; however, a semiconductordevice of one embodiment of the present invention is not limitedthereto. For example, a semiconductor region having a protruding portionmay be formed by processing an SOI substrate.

Structure Example 3 of Semiconductor Device

A semiconductor device 1200 in FIG. 4 is different from thesemiconductor device 1000 in FIG. 2 in that the first transistor 720 andthe power storage element 740 are provided below the second transistor730, and that the first transistor 720 and the power storage element 740do not overlap with each other.

For the semiconductor device 1200 having the structure shown in FIG. 4,plugs and wirings connected to the first transistor 720 can be formed atthe same time as plugs and wirings connected to the power storageelement 740, whereby the process can be simplified. Furthermore, in thecase where formation of the power storage element 740 requires heattreatment at such a high temperature as will destroy the plug 701 or thewiring 705, the structure shown in FIG. 4 is preferable because the plug701 and the wiring 705 can be formed after the power storage element 740is formed.

In FIG. 4, the power storage element 740 is formed after the firsttransistor 720 is formed; however, the first transistor 720 may beformed after the power storage element 740 is formed first. In the casewhere formation of the power storage element 740 requires heat treatmentat such a high temperature as will destroy the first transistor 720 inparticular, it is preferable to form the power storage element 740 firstand then form the first transistor 720.

For the other components of the semiconductor device 1200, thedescription of the semiconductor device 1000 is referred to.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 2

In this embodiment, the detail and structural examples of an electricdouble-layer capacitor (EDLC) that can be used for the power storageelement mentioned in Embodiment 1 will be described with reference todrawings.

Structural Example 1 of Power Storage Element

FIG. 5A is a top view of an EDLC 200, and FIG. 5B shows across-sectional view taken along the dashed-dotted line X-Y in FIG. 5A.In FIG. 5A, some components are increased or reduced in size, or omittedfor easy understanding.

The EDLC 200 in FIGS. 5A and 5B includes an insulating film 201, acurrent collector layer 202 formed over the insulating film 201, anactive material layer 203 formed over the current collector layer 202,an electrolyte layer 204 formed over the active material layer 203, anactive material layer 205 formed over the electrolyte layer 204, and acurrent collector layer 206 formed over the active material layer 205.The current collector layer 202 and the active material layer 203 have afunction of one of a positive electrode and a negative electrode, andthe active material layer 205 and the current collector layer 206 have afunction of the other of the positive electrode and the negativeelectrode. In addition, an insulating film 207 is formed at least overthe current collector layer 206, and a wiring 208 is formed in anopening portion of the insulating film 207. The wiring 208 iselectrically connected to the current collector layer 206.

The insulating film 201 may be formed as a single layer or a stackedlayer using at least one of the following materials: silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, aluminum nitride, aluminum oxynitride, hafnium oxide, zirconiumoxide, yttrium oxide, gallium oxide, lanthanum oxide, cesium oxide,tantalum oxide, and magnesium oxide.

The current collector layer 202 and the current collector layer 206 canbe formed by a sputtering method, a CVD method, a nanoimprint method, anevaporation method, a coating method, or the like. When a sputteringmethod is used, it is preferable to use a DC power supply rather than anRF power supply for deposition. A sputtering method using a DC powersupply is preferable because the deposition rate is high and thus cycletime is short. The thickness of each of the current collector layers 202and 206 may be greater than or equal to 100 nm and less than or equal to100 μm, for example.

For the current collector layers 202 and 206, a conductive material orthe like can be used, for example. The current collector layers 202 and206 may be each a single layer or a stacked layer formed using, forexample, one or more of the following as the conductive material: gold,platinum, zinc, iron, nickel, copper, aluminum, titanium, tantalum, andmanganese. Alternatively, a single-layer or stacked-layer conductivefilm including an alloy of the above metals or a compound containing anyof these as a main component may be used. Alternatively, the currentcollector layers 202 and 206 can be formed using an aluminum alloy towhich an element that improves heat resistance, such as silicon,neodymium, scandium, or molybdenum, is added. Still alternatively, ametal element which forms silicide by reacting with silicon can be used.Examples of the metal element which forms silicide by reacting withsilicon include zirconium, titanium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, cobalt, nickel, and the like.

For the active material layers 203 and 205, a material with a highspecific surface area, onto and from which ions can be adsorbed anddesorbed, may be used. For example, a carbon-based material can be used.Examples of a carbon-based material include activated carbon, graphite,graphitizing carbon (soft carbon), non-graphitizing carbon (hardcarbon), a carbon nanotube, graphene, and carbon black. Examples ofgraphite include artificial graphite such as meso-carbon microbeads(MCMB), coke-based artificial graphite, or pitch-based artificialgraphite and natural graphite such as spherical natural graphite.

Furthermore, it is possible to use the above material, onto and fromwhich ions can be adsorbed and desorbed, for one of the active materiallayers 203 and 205 and use a material, into and from which ions can beinserted and extracted, for the other of the active material layers 203and 205. As examples of a material into and from which ions can beinserted and extracted, the above carbon-based materials can be given.

It is also possible to use the above material, onto and from which ionscan be adsorbed and desorbed, for one of the active material layers 203and 205 and use a material, which can be alloyed and dealloyed withlithium ions, for the other of the active material layers 203 and 205.As a metal that can be alloyed and dealloyed with lithium ions, amaterial containing at least one of Ga, Si, Al, Ge, Sn, Pb, Sb, Bi, Ag,Zn, Cd, In, and the like can be used, for example. Examples of thealloy-based material using such elements include Mg₂Si, Mg₂Ge, Mg₂Sn,SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃,LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, and SbSn. Further, an oxide such as SiO,SnO, or SnO₂ may also be used for the other of the active materiallayers 203 and 205.

Further, the active material layers 203 and 205 may each include abinder for increasing adhesion of the active material.

It is preferable for the binder to include, for example, water-solublepolymers. As the water-soluble polymers, a polysaccharide or the likecan be used. As the polysaccharide, a cellulose derivative such ascarboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose,starch, or the like can be used.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, or ethylene-propylene-diene copolymer is preferablyused. Any of these rubber materials is more preferably used incombination with the aforementioned water-soluble polymers.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, isobutylene,polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF), orpolyacrylonitrile (PAN) can be preferably used.

Two or more of the above materials may be used in combination for thebinder.

Furthermore, the active material layers 203 and 205 may each include aconductive additive or the like for improving the conductivity of theactive material layers.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (acetylene black (AB)) and graphene. Alternatively, metalpowder or metal fibers of copper, nickel, aluminum, silver, gold, or thelike, a conductive ceramic material, or the like can be used.

Flaky graphene has an excellent electrical characteristic of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength. Thus, the use of graphene as the conductiveadditive can increase contact points and the contact area of activematerials.

Note that graphene in this specification includes single-layer grapheneand multilayer graphene including two to hundred layers. Single-layergraphene refers to a one-atom-thick sheet of carbon molecules having itbonds. Graphene oxide refers to a compound formed by oxidation of suchgraphene. When graphene oxide is reduced to form graphene, oxygencontained in the graphene oxide is not entirely released and part of theoxygen remains in the graphene. In the case where graphene containsoxygen, the proportion of the oxygen measured by X-ray photoelectronspectroscopy (XPS) is greater than or equal to 2% and less than or equalto 20%, preferably greater than or equal to 3% and less than or equal to15% of the whole graphene.

The film thickness of each of the active material layers 203 and 205 maybe greater than or equal to 100 nm and less than or equal to 100 μm, forexample.

The electrolyte layer 204 preferably includes a solid electrolyte thatcan be formed by a sputtering method, an evaporation method, a CVDmethod, a laser ablation method, a gas deposition method, a coatingmethod, a sol-gel method, or the like.

A sulfide-based solid electrolyte can be used for the electrolyte layer204. Examples of a sulfide-based solid electrolyte are lithium complexsulfide materials such as Li₇P₃S₁₁, Li_(3.25)P_(0.95)S₄, Li₁₀GeP₂S₁₂,Li_(3.25)Ge_(0.25)P_(0.75)S₄, Li₂S—P₂S₅, Li₂S—GeS₂, Li₂S—SiS₂—Li₃PO₄,Li₂S—SiS₂—Ga₂S₃, Li₂S—SiS₂—Li₄SiO₄, LiI—Li₂S—P₂S₅, LiI—Li₂S—B₂S₃, andLiI—Li₂S—SiS₂.

An oxide-based solid electrolyte can also be used as the electrolytelayer 204. Examples of an oxide-based solid electrolyte includeLi_(1.3)Al_(0.3)Ti_(1.7)(PO₄)₃, Li_(1.07)Al_(0.69)Ti_(1.4)(PO₄)₃,Li₄SiO₄—Li₃BO₃, Li_(2.9)PO_(3.3)N_(0.46), Li_(3.6)Si_(0.6)P_(0.4)O₄,Li_(1.5)Al_(0.5)Ge_(1.6)(PO₄)₃, Li₂O, Li₂CO₃, Li₂MoO₄, Li₃PO₄, Li₃VO₄,Li₄SiO₄, LLT (La_(2/3-x)Li_(3x)TiO₃), and LLZ (Li₇La₃Zr₂O₁₂).

A solid electrolyte containing Li⁺ and BH₄ ⁻ may also be used for theelectrolyte layer 204. For example, solid electrolytes such as LiBH₄,Li(BH₄)_(0.75)I_(0.25), and Li(BH₄)_(0.75)Br_(0.25) may be used.

Alternatively, a polymer solid electrolyte such as poly(ethylene oxide)(PEO) formed by a coating method or the like may be used for theelectrolyte layer 204. Still alternatively, a composite solidelectrolyte containing any of the above inorganic solid electrolytes anda polymer solid electrolyte may be used.

The thickness of the electrolyte layer 204 may be, for example, greaterthan or equal to 100 nm and less than or equal to 100 μm.

The insulating film 207 has a function of protecting the EDLC 200. Asthe insulating film 207, for example, an insulating material such as aresin (e.g., a polyimide resin, a polyamide resin, an acrylic resin, asiloxane resin, an epoxy resin, or a phenol resin), glass, an amorphouscompound, or ceramics can be used. Furthermore, a layer containingcalcium fluoride or the like may be provided as a water absorption layerbetween resin layers. The insulating film 207 can be formed by a spincoating method, an ink jet method, or the like. Alternatively, theinsulating film 207 can be formed to have a single-layer structure or astacked-layer structure using one or more of silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,aluminum nitride, aluminum oxynitride, hafnium oxide, zirconium oxide,yttrium oxide, gallium oxide, lanthanum oxide, cesium oxide, tantalumoxide, and magnesium oxide.

The wiring 208 preferably has a single-layer structure or astacked-layer structure of a conductive film containing a low-resistancematerial selected from copper (Cu), tungsten (W), molybdenum (Mo), gold(Au), aluminum (Al), manganese (Mn), titanium (Ti), tantalum (Ta),nickel (Ni), chromium (Cr), lead (Pb), tin (Sn), iron (Fe), and cobalt(Co), an alloy of such a low-resistance material, or a compoundcontaining such a material as its main component.

A separator 209 may be provided in the electrolyte layer 204 to preventthe positive electrode and the negative electrode from beingshort-circuited, as shown in FIG. 5C. As the separator 209, an insulatorwith pores is preferably used. For example, cellulose, polypropylene,polyethylene, and the like may be used.

Furthermore, in accordance with the amount of electric power requiredfor a semiconductor device or electronic device connected to the EDLC, aplurality of EDLCs 200 may be connected in series and/or in parallel. Inparticular, connecting a plurality of stacked EDLCs 200 in series and/orin parallel is preferable because the energy density of the EDLC can beincreased while the area occupied by the EDCL can be reduced.

Fabricating the EDLC 200 with the above-described structures leads to asafer, more reliable power storage element. In addition, a minute powerstorage element having a high affinity for a semiconductor element canbe provided.

Structural Example 2 of Power Storage Element

FIG. 6A is a top view of an EDLC 210, and FIG. 6B shows across-sectional view taken along the dashed-dotted line X-Y in FIG. 6A.In FIG. 6A, some components are increased or reduced in size, or omittedfor easy understanding.

The EDLC 210 in FIGS. 6A and 6B is different from the EDLC 200 in FIGS.5A and 5B in that an electrolyte layer 204, an active material layer205, and a current collector layer 206 are formed after an electrolytelayer 202 and an active material layer 203 are processed to take islandshapes. Such a structure can prevent a positive electrode and a negativeelectrode from being short-circuited. In addition, the EDLC 210 isdifferent from the EDLC 200 in that the current collector layer 206 isused as a lead wiring.

A separator 209 may be provided in the electrolyte layer 204 to preventthe positive electrode and the negative electrode from beingshort-circuited, as shown in FIG. 6C. As the separator 209, an insulatorwith pores is preferably used. For example, cellulose, polypropylene,polyethylene, and the like may be used.

For the details of the other components, the description regarding theEDLC 200 in FIGS. 5A to 5C is referred to.

Fabricating the EDLC 210 with the above-described structures leads to asafer, more reliable power storage element. In addition, a minute powerstorage element having a high affinity for a semiconductor element canbe provided.

Structural Example 3 of Power Storage Element

FIG. 7A is a top view of an EDLC 220, and FIG. 7B shows across-sectional view taken along the dashed-dotted line X-Y in FIG. 7A.In FIG. 7A, some components are increased or reduced in size, or omittedfor easy understanding.

The EDLC 220 in FIGS. 7A and 7B includes an insulating film 201, acurrent collector layer 202 formed over the insulating film 201, anactive material layer 213 formed over the current collector layer 202, aseparator 214 formed over the active material layer 213, an activematerial layer 215 formed over the separator 214, and a currentcollector layer 206 formed over the active material layer 215. Thecurrent collector layer 202 and the active material layer 213 have afunction of one of a positive electrode and a negative electrode, andthe active material layer 215 and the current collector layer 206 have afunction of the other of the positive electrode and the negativeelectrode. In addition, an insulating film 217 is formed at least overthe current collector layer 206, and a wiring 208 is formed in anopening portion of the insulating film 217. The wiring 208 iselectrically connected to the current collector layer 206.

It is preferable that each of the active material layer 213, theseparator 214, and the active material layer 215 be made up of aparticulate substance and include an ionic liquid.

Note that in FIG. 7B, the active material layer 213, the separator 214,and the active material layer 215 are shown with collections of circlesin order to schematically show that each layer is made up of aparticulate substance. The numbers, sizes, and shapes of the circles inthe figure do not reflect the numbers, sizes, and shapes of the actualparticles.

Next, the fabrication method of the EDLC 220 will be described withreference to FIGS. 8A to 8D.

First, the current collector layer 202, the active material layer 213,the separator 214, the active material layer 215, and the currentcollector layer 206 are formed or deposited over the insulating film 201(FIG. 8A).

The insulating film 201 and the current collector layers 202 and 206 canbe formed by a sputtering method, a CVD method, a nanoimprint method, anevaporation method, a coating method, or the like.

For materials that can be used for the insulating film 201 and thecurrent collector layers 202 and 206, the description regarding the EDLC200 in FIGS. 5A to 5C is referred to.

Porous materials are preferably used for the active material layers 213and 215. As the materials, carbon-based materials such as activatedcarbon and graphite can be given, for example.

The active material layers 213 and 215 may each include theabove-described binders and conductive additives.

An insulating material is preferably used for the separator 214. As theinsulating material, silicon oxide or the like can be given, forexample.

Particles that constitute the active material layers 213 and 215 and theseparator 214 are preferably deposited using a gas deposition method orgas aerosolization deposition method.

Then, the current collector layer 202, the active material layer 213,the separator 214, the active material layer 215, and the currentcollector layer 206 are processed into an island shape byphotolithography (FIG. 8B). Note that the shape shown in FIG. 8B may beobtained with the use of a shadow mask, electron beam exposure, or thelike instead of photolithography.

Then, an electrolytic solution 216 is dropped such that the activematerial layer 213, the separator 214, and the active material layer 215are soaked in the electrolytic solution 216 (FIG. 8C).

The electrolytic solution 216 has a function of an electrolyte of theEDLC 220. The electrolytic solution 216 preferably contains an ionicliquid (also referred to as a room temperature molten salt) that hasnon-flammability and non-volatility. Either one kind of ionic liquid ora combination of some kinds of ionic liquids is used. The use of theelectrolytic solution 216 containing an ionic liquid can prevent a powerstorage element from exploding or catching fire even when the powerstorage element internally shorts out or the internal temperatureincreases owing to overcharging or the like. An ionic liquid is composedof a cation and an anion. The ionic liquid of one embodiment of thepresent invention includes an organic cation and an anion. Examples ofthe organic cation include aromatic cations such as an imidazoliumcation and a pyridinium cation and aliphatic onium cations such as aquaternary ammonium cation, a tertiary sulfonium cation, and aquaternary phosphonium cation. Examples of the anion include amonovalent amide-based anion, a monovalent methide-based anion, afluorosulfonate anion, a perfluoroalkylsulfonate anion,tetrafluoroborate, perfluoroalkylborate, hexafluorophosphate, andperfluoroalkylphosphate.

As an example of the imidazolium cation, an ethylmethylimidazolium (EMI)cation can be given.

As an example of the quaternary ammonium cation, anN-methyl-N-propylpyrrolidinium (P13) cation or anN-methyl-N-propylpiperidinium (PP13) cation can be given.

Furthermore, an aprotic organic solvent may be mixed into any of theabove ionic liquids. As the aprotic organic solvent, for example, one ofethylene carbonate (EC), propylene carbonate (PC), butylene carbonate,chloroethylene carbonate, vinylene carbonate, γ-butyrolactone,γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC),ethyl methyl carbonate (EMC), methyl formate, methyl acetate, methylbutyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethylsulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile,tetrahydrofuran, sulfolane, and sultone can be used, or two or more ofthese solvents can be used in an appropriate combination in anappropriate ratio.

Furthermore, an additive agent such as vinylene carbonate, propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC),or LiBOB may be added to the electrolytic solution 216. Theconcentration of such an additive agent in the whole solvent is, forexample, higher than or equal to 0.1 wt % and lower than or equal to 5wt %.

Furthermore, a solute containing lithium ions may be added to theelectrolytic solution 216. As the solute, for example, one or plurallithium salts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN,LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃,LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂)(CF₃SO₂), andLiN(C₂F₅SO₂)₂, can be used in an appropriate combination and in anappropriate ratio.

The electrolytic solution 216 is preferably highly purified and containsa small amount of dust particles and elements other than the constituentelements of the electrolytic solution (hereinafter, also simply referredto as impurities). Specifically, the weight ratio of impurities to theelectrolyte solution is less than or equal to 1%, preferably less thanor equal to 0.1%, and more preferably less than or equal to 0.01%.

The process corresponding to FIG. 8C is preferably performed under areduced pressure atmosphere. A reduced pressure atmosphere allows gassescontained in the active material layer 213, the separator 214, and theactive material layer 215 to be released outside the element and theelectrolytic solution 216 to penetrate into the element owing tocapillary action.

Since the electrolytic solution 216 contains an ionic liquid, it hashigh viscosity, and it remains inside the element without leaking outeven under a reduced pressure.

Lastly, the insulating film 217 and the wiring 208 are formed (FIG. 8D).

For the wiring 208, the description of the EDLC 200 in FIGS. 5A to 5C isreferred to.

The insulating film 217 functions as a protective layer of the EDLC 220.The oxide insulating film 217 can be formed using an inorganicinsulating film containing one or more of aluminum oxide, aluminumnitride oxide, magnesium oxide, silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide,yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide,hafnium oxide, tantalum oxide, and the like. Alternatively, an organicresin such as a polyimide resin, a polyamide resin, an acrylic resin, asiloxane resin, an epoxy resin, or a phenol resin can also be used.

In particular, it is preferable that the insulating film 217 not reactwith the electrolytic solution 216. Therefore, among the above-listedinsulators, any of the inorganic insulating films is preferably used asthe insulating film 217.

The electrolytic solution 216 containing an ionic liquid has lowvolatility, and does not react with water to deteriorate. Therefore, aslong as the EDLC 220 does not include another material that reacts withwater to deteriorate, the EDLC 220 may be exposed to the air. In thatcase, the insulating film 217 need not be provided.

In the case where the EDLC 220 contains a material that reacts withwater to deteriorate, the EDLC 220 is placed in an environment where areduced pressure is maintained, whereby long-term reliability of theEDLC 220 can be obtained even without the insulating film 217.

Fabricating the EDLC 220 with the above-described structures leads to asafer, more reliable power storage element. In addition, a minute powerstorage element having a high affinity for a semiconductor element canbe provided.

Structural Example 4 of Power Storage Element

FIG. 9A is a top view of an EDLC 230, and FIG. 9B shows across-sectional view taken along the dashed-dotted line X-Y in FIG. 9A.In FIG. 9A, some components are increased or reduced in size, or omittedfor easy understanding.

The EDCL 230 in FIGS. 9A and 9B is different from the EDLC 220 in FIGS.7A and 7B in that a separator 214, an active material layer 215, and acurrent collector layer 206 are formed after a current collector layer202 and an active material layer 213 are processed into an island shape.Such a structure can prevent a positive electrode and a negativeelectrode from being short-circuited. In addition, the EDLC 230 isdifferent from the EDLC 220 in that the current collector layer 206 isused as a lead wiring. Such a structure can simplify the fabricationprocess.

For the other components, the description regarding the EDLC 220 inFIGS. 7A and 7B is referred to.

Fabricating the EDLC 230 with the above-described structures leads to asafer, more reliable power storage element. In addition, a minute powerstorage element having a high affinity for a semiconductor element canbe provided.

Structural Example 5 of Power Storage Element

An EDLC 240 in FIG. 10A includes an insulating film 201, a currentcollector layer 202 formed over the insulating film 201, an activematerial layer 203 formed over the current collector layer 202, anelectrolyte layer 204 formed over the active material layer 203, aninsulating film 251 formed over the electrolyte layer 204, an activematerial layer 205 formed over the electrolyte layer 204 and theinsulating film 251, and a current collector layer 206 formed over theactive material layer 205. The current collector layer 202 and theactive material layer 203 function as one of a positive electrode and anegative electrode, and the current collector layer 206 and the activematerial layer 205 function as the other of the positive electrode andthe negative electrode. Further, an insulating film 207 is formed atleast over the current collector layer 206.

In the EDLC 240 in FIG. 10A, the insulating film 251 provided betweenthe electrolyte layer 204 and the active material layer 205 can preventthe positive electrode and the negative electrode from beingshort-circuited.

The insulating film 251 can be formed using, for example, an organicresin or an inorganic insulating material. As the organic resin, forexample, a polyimide resin, a polyamide resin, an acrylic resin, asiloxane resin, an epoxy resin, or a phenol resin can be used. As theinorganic insulating material, silicon oxide, silicon oxynitride, or thelike can be used. In particular, a photosensitive resin is preferablyused for easy formation of the insulating film 251. There is noparticular limitation on the method for forming the insulating film 251.A photolithography method, a sputtering method, an evaporation method, adroplet discharging method (e.g., an inkjet method), a printing method(e.g., a screen printing method or an offset printing method), or thelike can be used.

For the details of the other components of the EDLC 240, the descriptionof the EDLC 200 in FIGS. 5A to 5C is referred to.

In the EDLC 240, the insulating film 251 may be formed over the activematerial layer 203, as shown in FIG. 10B.

Fabricating the EDLC 240 with the above-described structures leads to asafer, more reliable power storage element. In addition, a minute powerstorage element having a high affinity for a semiconductor element canbe provided.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 3

In this embodiment, an example of a transistor which can be used as thetransistor 730 described in Embodiment 1 will be described.

Structural Example 1 of Transistor

FIGS. 11A to 11D are a top view and cross-sectional views whichillustrate the transistor 730. FIG. 11A is the top view. FIG. 11Bcorresponds to a cross section taken along the dashed-dotted line Y1-Y2in FIG. 11A. FIG. 11C corresponds to a cross section taken along thedashed-dotted line X1-X2 in FIG. 11A. FIG. 11D corresponds to a crosssection taken along the dashed-dotted line X3-X4 in FIG. 11A. In FIGS.11A to 11D, some components are increased or reduced in size, or omittedfor easy understanding. In some cases, the direction of thedashed-dotted line Y1-Y2 is referred to as a channel length directionand the direction of the dashed-dotted line X1-X2 is referred to as achannel width direction.

Note that a channel length refers to, for example, a distance between asource (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor (or aportion where current flows in a semiconductor when a transistor is on)and a gate electrode overlap with each other or a region where a channelis formed in a top view of the transistor. In one transistor, channellengths in all regions are not necessarily the same. In other words, achannel length of one transistor is not limited to one value in somecases. Therefore, in this specification, a channel length is any one ofvalues, the maximum value, the minimum value, or the average value in aregion where a channel is formed.

A channel width refers to, for example, the length of a portion where asource and a drain face each other in a region where a semiconductor (ora portion where current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other, or a region where achannel is formed. In one transistor, channel widths in all regions donot necessarily the same. In other words, a channel width of onetransistor is not limited to one value in some cases. Therefore, in thisspecification, a channel width is any one of values, the maximum value,the minimum value, or the average value in a region where a channel isformed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in a minutetransistor having a three-dimensional structure, the proportion of achannel region formed in a side surface of a semiconductor is higherthan the proportion of a channel region formed in a top surface of asemiconductor in some cases. In that case, an effective channel widthobtained when a channel is actually formed is greater than an apparentchannel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example, toestimate an effective channel width from a design value, it is necessaryto assume that the shape of a semiconductor is known as an assumptioncondition. Therefore, in the case where the shape of a semiconductor isnot known accurately, it is difficult to measure an effective channelwidth accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Further, in this specification, in the casewhere the term “channel width” is simply used, it may denote asurrounded channel width or an apparent channel width. Alternatively, inthis specification, in the case where the term “channel width” is simplyused, it may denote an effective channel width. Note that the values ofa channel length, a channel width, an effective channel width, anapparent channel width, a surrounded channel width, and the like can bedetermined by obtaining and analyzing a cross-sectional TEM image andthe like.

Note that in the case where electric field mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from one in the case where an effective channelwidth is used for the calculation is obtained in some cases.

The transistor 730 includes a substrate 640; an insulating film 651 overthe substrate 640; a conductive film 674 formed over the insulating film651; an insulating film 656 formed over the insulating film 651 and theconductive film 674; an insulating film 652 formed over the insulatingfilm 656; a semiconductor 661 and a semiconductor 662 stacked over theinsulating film 652 in this order; a conductive film 671 and aconductive film 672 in contact with a top surface of the semiconductor662; a semiconductor 663 in contact with the semiconductor 661, thesemiconductor 662, the conductive film 671, and the conductive film 672;an insulating film 653 and a conductive film 673 over the semiconductor663; an insulating film 654 over the conductive film 673 and theinsulating film 653; and an insulating film 655 over the insulating film654. Note that the semiconductor 661, the semiconductor 662, and thesemiconductor 663 are collectively referred to as a semiconductor 660.

The conductive film 671 has a function of a source electrode of thetransistor 730. The conductive film 672 has a function of a drainelectrode of the transistor 730.

The conductive film 673 has a function of a first gate electrode of thetransistor 730.

The insulating film 653 has a function of a first gate insulating filmof the transistor 730.

The conductive film 674 has a function of a second gate electrode of thetransistor 730.

The insulating film 656 and the insulating film 652 have a function of asecond gate insulating film of the transistor 730.

The conductive film 674 and the conductive film 673 may be supplied withdifferent potentials, or supplied with the same potentials at the sametime. Further, the conductive film 674 may be omitted in some cases.

As illustrated in FIG. 11C, a side surface of the semiconductor 662 issurrounded by the conductive film 673. With such a structure, thesemiconductor 662 can be electrically surrounded by an electric field ofthe conductive film 673 (a structure in which a semiconductor iselectrically surrounded by an electric field of a conductive film (gateelectrode) is referred to as a surrounded channel (s-channel)structure). Therefore, a channel is formed in the entire semiconductor662 (bulk) in some cases. In the s-channel structure, a large amount ofcurrent can flow between a source and a drain of a transistor, so thathigh current in an on state (on-state current) can be achieved. Thes-channel structure enables a transistor to operate at high frequency.

The s-channel structure, because of its high on-state current, issuitable for a semiconductor device such as large-scale integration(LSI) which requires a miniaturized transistor. A semiconductor deviceincluding the miniaturized transistor can have a high integration degreeand high density. The transistor preferably has, for example, a regionwhere a channel length is greater than or equal to 10 nm and less than 1μm, more preferably greater than or equal to 10 nm and less than 100 nm,still more preferably greater than or equal to 10 nm and less than 60nm, and yet still more preferably greater than or equal to 10 nm andless than 30 nm.

Since high on-state current can be obtained, the s-channel structure issuitable for a transistor that needs to operate at high frequency. Asemiconductor device including the transistor can operate at highfrequency.

In addition, the s-channel structure is suitable for a power controltransistor because of its high on-state current. To employ the s-channelstructure in the power control transistor that requires a high withstandvoltage, the channel length is preferably long. For example, thetransistor preferably has a region where the channel length is greaterthan or equal to 1 μm, more preferably greater than or equal to 10 μm,and still more preferably greater than or equal to 100 μm.

<Substrate>

As the substrate 640, an insulator substrate, a semiconductor substrate,or a conductor substrate may be used, for example. As the insulatorsubstrate, a glass substrate, a quartz substrate, a sapphire substrate,a stabilized zirconia substrate (e.g., an yttria-stabilized zirconiasubstrate), or a resin substrate is used, for example. As thesemiconductor substrate, a semiconductor substrate of silicon,germanium, or the like, or a compound semiconductor substrate of siliconcarbide, silicon germanium, gallium arsenide, indium phosphide, zincoxide, or gallium oxide can be used, for example. A semiconductorsubstrate in which an insulator region is provided in the abovesemiconductor substrate, e.g., a silicon on insulator (SOI) substrate orthe like is used. As the conductor substrate, a graphite substrate, ametal substrate, an alloy substrate, a conductive resin substrate, orthe like is used. A substrate including a metal nitride, a substrateincluding a metal oxide, or the like is used. An insulator substrateprovided with a conductor or a semiconductor, a semiconductor substrateprovided with a conductor or an insulator, a conductor substrateprovided with a semiconductor or an insulator, or the like is used.Alternatively, any of these substrates over which an element is providedmay be used. As the element provided over the substrate, a capacitor, aresistor, a switching element, a light-emitting element, a memoryelement, or the like is used.

A flexible substrate may be used as the substrate 640. As a method forproviding a transistor over a flexible substrate, there is a method inwhich a transistor is formed over a non-flexible substrate, and then thetransistor is separated and transferred to the substrate 640 that is aflexible substrate. In that case, a separation layer is preferablyprovided between the non-flexible substrate and the transistor. As thesubstrate 640, a sheet, a film, or foil containing a fiber may be used.The substrate 640 may have elasticity. The substrate 640 may have aproperty of returning to its original shape when bending or pulling isstopped. Alternatively, the substrate 640 may have a property of notreturning to its original shape. The thickness of the substrate 640 is,for example, greater than or equal to 5 μm and less than or equal to 700μm, preferably greater than or equal to 10 μm and less than or equal to500 μm, more preferably greater than or equal to 15 μm and less than orequal to 300 μm. When the substrate 640 has small thickness, the weightof the semiconductor device can be reduced. When the substrate 640 hassmall thickness, even in the case of using glass or the like, thesubstrate 640 may have elasticity or a property of returning to itsoriginal shape when bending or pulling is stopped. Therefore, an impactapplied to the semiconductor device over the substrate 640, which iscaused by dropping or the like, can be reduced. That is, a durablesemiconductor device can be provided.

For the flexible substrate 640, metal, an alloy, resin, glass, or fiberthereof can be used, for example. The flexible substrate 640 preferablyhas a lower coefficient of linear expansion because deformation due toan environment is suppressed. The flexible substrate 640 is formedusing, for example, a material whose coefficient of linear expansion islower than or equal to 1×10⁻³/K, lower than or equal to 5×10⁻⁵/K, orlower than or equal to 1×10⁻⁵/K. Examples of the resin includepolyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide,polycarbonate, acrylic, and polytetrafluoroethylene (PTFE). Inparticular, aramid is preferably used for the flexible substrate 640because of its low coefficient of linear expansion.

<Base Insulating Film>

The insulating film 651 has a function of electrically isolating thesubstrate 640 and the conductive film 674 from each other.

As a material for the insulating film 651, a material containing siliconoxide, silicon nitride, silicon oxynitride, or silicon nitride oxide ispreferably used. Alternatively, a metal oxide such as aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, or hafnium oxynitride can be used.Note that in this specification, “oxynitride” refers to a material thatcontains oxygen at a higher proportion than nitrogen, and “nitrideoxide” refers to a material that contains nitrogen at a higherproportion than oxygen.

Alternatively, silicon oxide with favorable step coverage which isformed through reaction between TEOS, silane, or the like and oxygen,nitrous oxide, or the like may be used as the insulating film 651.

The insulating film 651 may be formed by a sputtering method, a chemicalvapor deposition (CVD) method (including a thermal CVD method, a metalorganic CVD (MOCVD) method, a plasma enhanced CVD (PECVD) method, andthe like), a molecular beam epitaxy (MBE) method, an atomic layerdeposition (ALD) method, a pulsed laser deposition (PLD) method, or thelike. In particular, it is preferable that the insulating film be formedby a CVD method, further preferably a plasma CVD method because coveragecan be further improved. It is preferable to use a thermal CVD method,an MOCVD method, or an ALD method in order to reduce plasma damage.

In the case where a semiconductor substrate is used as the substrate640, the insulating film 651 may be a thermal oxide film.

The conductive film 674 preferably has a single-layer structure or astacked-layer structure of a conductive film containing a low-resistancematerial selected from copper (Cu), tungsten (W), molybdenum (Mo), gold(Au), aluminum (Al), manganese (Mn), titanium (Ti), tantalum (Ta),nickel (Ni), chromium (Cr), lead (Pb), tin (Sn), iron (Fe), cobalt (Co),ruthenium (Ru), platinum (Pt), iridium (Ir), and strontium (Sr), analloy of such a low-resistance material, or a compound containing such amaterial as its main component. It is particularly preferable to use ahigh-melting-point material that has both heat resistance andconductivity, such as tungsten or molybdenum. In addition, theconductive film 674 is preferably formed using a low-resistanceconductive material such as aluminum or copper. The conductive film 674is preferably formed using a Cu—Mn alloy, since in that case, manganeseoxide formed at the interface with an insulator containing oxygen has afunction of preventing Cu diffusion.

The conductive film 674 can be formed by a sputtering method, a CVDmethod (including a thermal CVD method, an MOCVD method, a PECVD method,and the like), an MBE method, an ALD method, a PLD method, or the like.

The insulating film 652 preferably contains an oxide. In particular, theinsulating film 652 preferably contains an oxide material from whichpart of oxygen is released by heating. The insulating film 652preferably contains an oxide containing oxygen in excess of that in thestoichiometric composition. Part of oxygen is released due to heatingfrom an oxide film containing oxygen in excess of that in thestoichiometric composition. Oxygen released from the insulating film 652is supplied to the semiconductor 660 that is an oxide semiconductor, sothat oxygen vacancies in the oxide semiconductor can be reduced.Consequently, changes in the electrical characteristics of thetransistor can be reduced and the reliability of the transistor can beimproved.

The oxide film containing oxygen in excess of that in the stoichiometriccomposition is an oxide film of which the amount of released oxygenconverted into oxygen atoms is greater than or equal to 1.0×10¹⁸atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ inthermal desorption spectroscopy (TDS) analysis, for example. Note thatthe temperature of the film surface in the TDS analysis is preferablyhigher than or equal to 100° C. and lower than or equal to 700° C., orhigher than or equal to 100° C. and lower than or equal to 500° C.

For example, for the insulating film 652, a material containing siliconoxide or silicon oxynitride is preferably used. Alternatively, a metaloxide such as aluminum oxide, aluminum oxynitride, gallium oxide,gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, orhafnium oxynitride can be used.

To make the insulating film 652 contain excess oxygen, the insulatingfilm 652 may be formed in an oxygen atmosphere, for example.Alternatively, a region containing excess oxygen may be formed byintroducing oxygen into the insulating film 652 that has been formed.Both the methods may be combined.

For example, oxygen (including at least one of an oxygen radical, anoxygen atom, and an oxygen ion) is introduced into the insulating film652 that has been formed, so that a region containing excess oxygen isformed. Oxygen can be introduced by an ion implantation method, an iondoping method, a plasma immersion ion implantation method, plasmatreatment, or the like.

A gas containing oxygen can be used for oxygen introduction treatment.As the gas containing oxygen, oxygen, nitrous oxide, nitrogen dioxide,carbon dioxide, carbon monoxide, and the like can be used. Further, arare gas may be included in the gas containing oxygen for the oxygenintroduction treatment. Furthermore, hydrogen or the like may beincluded. For example, a mixed gas of carbon dioxide, hydrogen, andargon may be used.

After the insulating film 652 is formed, the insulating film 652 may besubjected to planarization treatment using a CMP method or the like toimprove the planarity the top surface thereof.

The insulating film 656 may be provided between the conductive film 674and the insulating film 652. The insulating film 656 has a function ofpreventing oxygen contained in the insulating film 652 from decreasingby bonding to metal contained in the conductive film 674.

The insulating film 656 preferably has a blocking effect against oxygen,hydrogen, water, alkali metal, alkaline earth metal, and the like. Theinsulating film 656 can be, for example, a nitride insulating film. Thenitride insulating film is formed using silicon nitride, silicon nitrideoxide, aluminum nitride, aluminum nitride oxide, or the like. An oxideinsulating film may be provided instead of the nitride insulating film.As the oxide insulating film having a blocking effect against oxygen,hydrogen, water, and the like, an aluminum oxide film, an aluminumoxynitride film, a gallium oxide film, a gallium oxynitride film, anyttrium oxide film, an yttrium oxynitride film, a hafnium oxide film,and a hafnium oxynitride film can be given.

<Semiconductor>

Next, a semiconductor that can be used as the semiconductor 661, thesemiconductor 662, the semiconductor 663, or the like will be described.

In the transistor 730, it is preferable that current flowing between asource and a drain in an off state (off-state current) be low. Here, theterm “low off-state current” means that a normalized off-state currentper micrometer of a channel width at room temperature with asource-drain voltage of 10 V is less than or equal to 10×10⁻²¹ A. Anexample of a transistor with such low off-state current is a transistorincluding an oxide semiconductor as a semiconductor.

The semiconductor 662 is an oxide semiconductor containing indium (In),for example. The semiconductor 662 can have high carrier mobility(electron mobility) by containing indium, for example. The semiconductor662 preferably contains an element M. The element M is preferablyaluminum (Al), gallium (Ga), yttrium (Y), tin (Sn), or the like. Otherelements that can be used as the element M are boron (B), silicon (Si),titanium (Ti), iron (Fe), nickel (Ni), germanium (Ge), zirconium (Zr),molybdenum (Mo), lanthanum (La), cerium (Ce), neodymium (Nd), hafnium(Hf), tantalum (Ta), tungsten (W), and the like. Note that two or moreof the above elements may be used in combination as the element M. Theelement M is an element having a high bonding energy with oxygen, forexample. The element M is an element whose bonding energy with oxygen ishigher than that of indium, for example. The element M is an elementthat can increase the energy gap of the oxide semiconductor, forexample. Furthermore, the semiconductor 662 preferably contains zinc(Zn). When the oxide semiconductor contains zinc, the oxidesemiconductor is easily crystallized in some cases.

Note that the semiconductor 662 is not limited to the oxidesemiconductor containing indium. The semiconductor 662 may be, forexample, an oxide semiconductor which does not contain indium andcontains zinc, an oxide semiconductor which does not contain indium andcontains gallium, or an oxide semiconductor which does not containindium and contains tin, e.g., a zinc tin oxide or a gallium tin oxide.

For the semiconductor 662, an oxide with a wide energy gap is used. Theenergy gap of the semiconductor 662 is, for example, greater than orequal to 2.5 eV and less than or equal to 4.2 eV, preferably greaterthan or equal to 2.8 eV and less than or equal to 3.8 eV, morepreferably greater than or equal to 3 eV and less than or equal to 3.5eV.

The semiconductor 662 is preferably a CAAC-OS film which will bedescribed later.

For example, the semiconductor 661 and the semiconductor 663 are oxidesemiconductors each including one or more, or two or more elements otherthan oxygen included in the semiconductor 662. Since the semiconductor661 and the semiconductor 663 each include one or more, or two or moreelements other than oxygen included in the semiconductor 662, aninterface state is less likely to be formed at the interface between thesemiconductor 661 and the semiconductor 662 and the interface betweenthe semiconductor 662 and the semiconductor 663.

In the case where the semiconductor 661 is an In-M-Zn oxide, when thetotal proportion of In and M is assumed to be 100 atomic %, theproportions of In and M are preferably set to be lower than 50 atomic %and higher than 50 atomic %, respectively, more preferably lower than 25atomic % and higher than 75 atomic %, respectively. In the case wherethe semiconductor 661 is formed by a sputtering method, it is preferableto use a sputtering target that satisfies the above composition. Forexample, as the atomic ratio of the sputtering target, In:M:Zn=1:3:2 ispreferably satisfied.

In the case where the semiconductor 662 is an In-M-Zn oxide, when thetotal proportion of In and M is assumed to be 100 atomic %, theproportions of In and M are preferably set to be higher than 25 atomic %and lower than 75 atomic %, respectively, more preferably higher than 34atomic % and lower than 66 atomic %, respectively. In the case where thesemiconductor 662 is formed by a sputtering method, it is preferable touse a sputtering target that satisfies the above composition. Forexample, as the atomic ratio of the sputtering target, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, In:M:Zn=2:1:3, In:M:Zn=3:1:2, or In:M:Zn=4:2:4.1 ispreferably satisfied. In particular, when a sputtering target with anatomic ratio of In to Ga and Zn of 4:2:4.1 is used, the atomic ratio ofIn to Ga and Zn in the semiconductor 662 may be 4:2:3 or in theneighborhood of 4:2:3.

In the case where the semiconductor 663 is an In-M-Zn oxide, when thetotal proportion of In and M is assumed to be 100 atomic %, theproportions of In and M are preferably set to be lower than 50 atomic %and higher than 50 atomic %, respectively, more preferably lower than 25atomic % and higher than 75 atomic %, respectively. The semiconductor663 may be an oxide that is the same type as that of the semiconductor661. Note that the semiconductor 661 and/or the semiconductor 663 neednot necessarily contain indium in some cases. For example, thesemiconductor 661 and/or the semiconductor 663 may be gallium oxide.

Next, a function and an effect of the semiconductor 660 in which thesemiconductors 661 to 663 are stacked will be described using an energyband diagram in FIG. 12B. FIG. 12A is an enlarged view of the channelportion of the transistor 730 in FIG. 11B. FIG. 12B shows an energy banddiagram of a portion along the chain line A1-A2 in FIG. 12A. That is,FIG. 12B shows the energy band structure of a channel region of thetransistor 730.

In FIG. 12B, Ec652, Ec661, Ec662, Ec663, and Ec653 indicate the energyat the bottom of the conduction band of the insulating film 652, thesemiconductor 661, the semiconductor 662, the semiconductor 663, and theinsulating film 653, respectively.

Here, a difference in energy between the vacuum level and the bottom ofthe conduction band (the difference is also referred to as “electronaffinity”) corresponds to a value obtained by subtracting an energy gapfrom a difference in energy between the vacuum level and the top of thevalence band (the difference is also referred to as an ionizationpotential). Note that the energy gap can be measured using aspectroscopic ellipsometer. The energy difference between the vacuumlevel and the top of the valence band can be measured using anultraviolet photoelectron spectroscopy (UPS) device.

Since the insulating films 652 and 653 are insulators, Ec652 and Ec653are closer to the vacuum level than Ec661 to Ec663 (i.e., the insulatingfilms 652 and 653 have lower electron affinity than the semiconductors661 to 663).

As the semiconductor 662, an oxide having an electron affinity higherthan those of the semiconductors 661 and 663 is used. For example, asthe semiconductor 662, an oxide having an electron affinity higher thanthose of the semiconductors 661 and 663 by greater than or equal to 0.07eV and less than or equal to 1.3 eV, preferably greater than or equal to0.1 eV and less than or equal to 0.7 eV, more preferably greater than orequal to 0.15 eV and less than or equal to 0.4 eV is used. Note that theelectron affinity refers to an energy gap between the vacuum level andthe bottom of the conduction band.

An indium gallium oxide has a small electron affinity and a highoxygen-blocking property. Therefore, the semiconductor 663 preferablyincludes an indium gallium oxide. The gallium atomic ratio [Ga/(In+Ga)]is, for example, higher than or equal to 70%, preferably higher than orequal to 80%, more preferably higher than or equal to 90%.

At this time, when gate voltage is applied, a channel is formed in thesemiconductor 662 having the highest electron affinity in thesemiconductors 661 to 663.

Here, in some cases, there is a mixed region of the semiconductors 661and 662 between the semiconductors 661 and 662. Furthermore, in somecases, there is a mixed region of the semiconductors 662 and 663 betweenthe semiconductors 662 and 663. The mixed region has a low density ofinterface states. For that reason, the stack of the semiconductors 661to 663 has a band structure where energy at each interface is changedcontinuously (continuous junction).

At this time, electrons move mainly in the semiconductor 662, not in thesemiconductors 661 and 663. As described above, when the interface statedensity at the interface between the semiconductors 661 and 662 and theinterface state density at the interface between the semiconductors 662and 663 are decreased, electron movement in the semiconductor 662 isless likely to be inhibited and the on-sate current of the transistorcan be increased.

As factors of inhibiting electron movement are decreased, the on-statecurrent of the transistor can be increased. For example, in the casewhere there is no factor of inhibiting electron movement, electrons areassumed to move efficiently. Electron movement is inhibited, forexample, in the case where physical unevenness in a channel region islarge.

To increase the on-state current of the transistor, for example, rootmean square (RMS) roughness with a measurement area of 1 μm×1 μm of atop surface or a bottom surface of the semiconductor 662 (a formationsurface; here, the semiconductor 661) is less than 1 nm, preferably lessthan 0.6 nm, more preferably less than 0.5 nm, still more preferablyless than 0.4 nm. The average surface roughness (also referred to as Ra)with the measurement area of 1 μm×1 μm is less than 1 nm, preferablyless than 0.6 nm, more preferably less than 0.5 nm, still morepreferably less than 0.4 nm. The maximum difference (also referred to asP−V) with the measurement area of 1 μm×1 μm is less than 10 nm,preferably less than 9 nm, more preferably less than 8 nm, still morepreferably less than 7 nm. RMS roughness, Ra, and P−V can be measuredusing a scanning probe microscope SPA-500 manufactured by SII NanoTechnology Inc.

The electron movement is also inhibited, for example, in the case wherethe density of defect states is high in a region where a channel isformed.

For example, in the case where the semiconductor 662 contains oxygenvacancies (V_(O)), donor levels are formed by entry of hydrogen intosites of oxygen vacancies in some cases. A state in which hydrogenenters sites of oxygen vacancies are denoted by V_(O)H in the followingdescription in some cases. V_(O)H is a factor of decreasing the on-statecurrent of the transistor because V_(O)H scatters electrons. Note thatsites of oxygen vacancies become more stable by entry of oxygen than byentry of hydrogen. Thus, by decreasing oxygen vacancies in thesemiconductor 662, the on-state current of the transistor can beincreased in some cases.

For example, the hydrogen concentration at a certain depth of thesemiconductor 662 or in a certain region of the semiconductor 662, whichis measured by secondary ion mass spectrometry (SIMS), is higher than orequal to 1×10¹⁶ atoms/cm³ and lower than or equal to 2×10²⁰ atoms/cm³,preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than orequal to 5×10¹⁹ atoms/cm³, more preferably higher than or equal to1×10¹⁶ atoms/cm³ and lower than or equal to 1×10¹⁹ atoms/cm³, still morepreferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than orequal to 5×10¹⁸ atoms/cm³.

To decrease oxygen vacancies in the semiconductor 662, for example,there is a method in which excess oxygen in the insulating film 652 ismoved to the semiconductor 662 through the semiconductor 661. In thiscase, the semiconductor 661 is preferably a layer having anoxygen-transmitting property (a layer through which oxygen istransmitted).

In the case where the transistor has an s-channel structure, a channelis formed in the entire semiconductor 662. Therefore, as thesemiconductor 662 has larger thickness, a channel region becomes larger.In other words, the thicker the semiconductor 662 is, the larger theon-state current of the transistor can be.

Moreover, the thickness of the semiconductor 663 is preferably as smallas possible to increase the on-state current of the transistor. Forexample, the semiconductor 663 has a region with a thickness of lessthan 10 nm, preferably less than or equal to 5 nm, more preferably lessthan or equal to 3 nm. Meanwhile, the semiconductor 663 has a functionof blocking entry of elements other than oxygen (such as hydrogen andsilicon) included in the adjacent insulator into the semiconductor 662where a channel is formed. Thus, the semiconductor 663 preferably has acertain thickness. For example, the semiconductor 663 has a region witha thickness of greater than or equal to 0.3 nm, preferably greater thanor equal to 1 nm, more preferably greater than or equal to 2 nm. Thesemiconductor 663 preferably has an oxygen blocking property to inhibitoutward diffusion of oxygen released from the insulating film 652 andthe like.

To improve reliability, preferably, the thickness of the semiconductor661 is large and the thickness of the semiconductor 663 is small. Forexample, the semiconductor 661 has a region with a thickness of greaterthan or equal to 10 nm, preferably greater than or equal to 20 nm, morepreferably greater than or equal to 40 nm, still more preferably greaterthan or equal to 60 nm. When the thickness of the semiconductor 661 ismade large, a distance from an interface between the adjacent insulatorand the semiconductor 661 to the semiconductor 662 in which a channel isformed can be large. However, to prevent the productivity of thesemiconductor device from being decreased, the semiconductor 661 has aregion with a thickness of, for example, less than or equal to 200 nm,preferably less than or equal to 120 nm, more preferably less than orequal to 80 nm.

For example, a region in which the concentration of silicon measured bySIMS is higher than or equal to 1×10¹⁶ atoms/cm³ and lower than 1×10¹⁹atoms/cm³, preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lowerthan 5×10¹⁸ atoms/cm³, more preferably higher than or equal to 1×10¹⁶atoms/cm³ and lower than 2×10¹⁸ atoms/cm³ is provided between thesemiconductors 661 and 662. Further, a region in which the concentrationof silicon measured by SIMS is higher than or equal to 1×10¹⁶ atoms/cm³and lower than 1×10¹⁹ atoms/cm³, preferably higher than or equal to1×10¹⁶ atoms/cm³ and lower than 5×10¹⁸ atoms/cm³, more preferably higherthan or equal to 1×10¹⁶ atoms/cm³ and lower than 2×10¹⁸ atoms/cm³ isprovided between the semiconductors 662 and 663.

It is preferable to reduce the concentration of hydrogen in thesemiconductors 661 and 663 in order to reduce the concentration ofhydrogen in the semiconductor 662. The semiconductors 661 and 663 eachhave a region in which the concentration of hydrogen measured by SIMS ishigher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to2×10²⁰ atoms/cm³, preferably higher than or equal to 1×10¹⁶ atoms/cm³and lower than or equal to 5×10¹⁹ atoms/cm³, more preferably higher thanor equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 1×10¹⁹atoms/cm³, still more preferably higher than or equal to 1×10¹⁶atoms/cm³ and lower than or equal to 5×10¹⁸ atoms/cm³. It is preferableto reduce the concentration of nitrogen in the semiconductors 661 and663 in order to reduce the concentration of nitrogen in thesemiconductor 662. The semiconductors 661 and 663 each have a region inwhich the concentration of nitrogen measured by SIMS is higher than orequal to 1×10¹⁶ atoms/cm³ and lower than 5×10¹⁹ atoms/cm³, preferablyhigher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to5×10¹⁸ atoms/cm³, more preferably higher than or equal to 1×10¹⁶atoms/cm³ and lower than or equal to 1×10¹⁸ atoms/cm³, still morepreferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than orequal to 5×10¹⁷ atoms/cm³.

The above three-layer structure is an example. For example, a two-layerstructure without the semiconductor 661 or 663 may be employed. Afour-layer structure in which any one of the semiconductors described asexamples of the semiconductors 661 to 663 is provided below or over thesemiconductor 661 or below or over the semiconductor 663 may beemployed. An n-layer structure (n is an integer of five or more) inwhich any one of the semiconductors described as examples of thesemiconductors 661 to 663 is provided at two or more of the followingpositions may be employed: over the semiconductor 661, below thesemiconductor 661, over the semiconductor 663, and below thesemiconductor 663.

<Conductive Film>

The conductive films 671 to 673 preferably have a single-layer structureor a stacked-layer structure of a conductive film containing alow-resistance material selected from copper (Cu), tungsten (W),molybdenum (Mo), gold (Au), aluminum (Al), manganese (Mn), titanium(Ti), tantalum (Ta), nickel (Ni), chromium (Cr), lead (Pb), tin (Sn),iron (Fe), cobalt (Co), ruthenium (Ru), platinum (Pt), iridium (Ir), andstrontium (Sr); an alloy of such a low-resistance material; or acompound containing such a material as its main component. It isparticularly preferable to use a high-melting-point material that hasboth heat resistance and conductivity, such as tungsten or molybdenum.In addition, the conductive film is preferably formed using alow-resistance conductive material such as aluminum or copper. Theconductive film is preferably formed using a Cu—Mn alloy, since in thatcase, manganese oxide formed at the interface with an insulatorcontaining oxygen has a function of preventing Cu diffusion.

The conductive films 671 and 672 are preferably formed using aconductive oxide including noble metal, such as iridium oxide, rutheniumoxide, or strontium ruthenate. Such a conductive oxide hardly takesoxygen from an oxide semiconductor even when it is in contact with theoxide semiconductor and hardly generates oxygen vacancies in the oxidesemiconductor.

The conductive films 671 to 673 can be formed by, for example, asputtering method, a CVD method (including a thermal CVD method, anMOCVD method, a PECVD method, and the like), an MBE method, an ALDmethod, or a PLD method.

<Gate Insulating Film>

The insulating film 653 can be formed using an insulating filmcontaining one or more of aluminum oxide, magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating film 653 may be a stack of any of the above materials. Notethat the insulating film 653 may contain lanthanum (La), nitrogen,zirconium (Zr), or the like as an impurity.

An example of a stacked-layer structure of the insulating film 653 willbe described. The insulating film 653 contains oxygen, nitrogen,silicon, or hafnium, for example. Specifically, the insulating film 653preferably includes hafnium oxide and silicon oxide or siliconoxynitride.

Hafnium oxide has a higher dielectric constant than silicon oxide andsilicon oxynitride. Therefore, by using hafnium oxide, the thickness ofthe insulating film 653 can be made larger than the case where siliconoxide is used; thus, leakage current due to tunnel current can be low.That is, it is possible to provide a transistor with a low off-statecurrent.

<Protective Insulating Film>

The insulating film 654 has a function of blocking oxygen, hydrogen,water, alkali metal, alkaline earth metal, and the like. The insulatingfilm 654 can prevent outward diffusion of oxygen from the semiconductor660 and entry of hydrogen, water, or the like into the semiconductor 660from the outside.

The insulating film 654 can be formed by a sputtering method, a CVDmethod (including a thermal CVD method, an MOCVD method, a PECVD method,and the like), an MBE method, an ALD method, a PLD method, or the like,for example. In particular, it is preferable that the insulating film beformed by a CVD method, more preferably a plasma CVD method becausecoverage can be further improved. It is preferable to use a thermal CVDmethod, an MOCVD method, or an ALD method in order to reduce plasmadamage.

The insulating film 654 can be a nitride insulating film, for example.The nitride insulating film is formed using silicon nitride, siliconnitride oxide, aluminum nitride, aluminum nitride oxide, or the like.Note that instead of the nitride insulating film, an oxide insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike, may be provided. As the oxide insulating film, an aluminum oxidefilm, an aluminum oxynitride film, a gallium oxide film, a galliumoxynitride film, an yttrium oxide film, an yttrium oxynitride film, ahafnium oxide film, and a hafnium oxynitride film can be given.

An aluminum oxide film is preferably used as the insulating film 654because it is highly effective in preventing transmission of both oxygenand impurities such as hydrogen and moisture. In addition, oxygencontained in the aluminum oxide film can be diffused into thesemiconductor 660.

After the insulating film 654 is formed, heat treatment is preferablyperformed. Through this heat treatment, oxygen can be supplied to thesemiconductor 660 from the insulating film 652 or the like; thus, oxygenvacancies in the semiconductor 660 can be reduced. Because oxygenreleased from the insulating film 652 is blocked by the insulating film656 and the insulating film 654 at this time, the oxygen can beeffectively confined. Thus, the amount of oxygen supplied to thesemiconductor 660 can be increased, so that oxygen vacancies in thesemiconductor 660 can be effectively reduced.

Next, the insulating film 655 is formed. The insulating film 655 can beformed by a sputtering method, a CVD method (including a thermal CVDmethod, an MOCVD method, a PECVD method, and the like), an MBE method,an ALD method, a PLD method, or the like. In particular, it ispreferable that the insulating film 655 be formed by a CVD method, morepreferably a plasma CVD method because coverage can be further improved.It is preferable to use a thermal CVD method, an MOCVD method, or an ALDmethod in order to reduce plasma damage. In the case where theinsulating film 655 is formed using an organic insulating material suchas an organic resin, a coating method such as a spin coating method maybe used. After the insulating film 655 is formed, a top surface thereofis preferably subjected to planarization treatment.

The insulating film 655 can be formed using an insulator containing oneor more materials selected from aluminum oxide, aluminum nitride oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,tantalum oxide, and the like. Alternatively, the insulating film 655 canbe formed using an organic resin such as a polyimide resin, a polyamideresin, an acrylic resin, a siloxane resin, an epoxy resin, or a phenolresin. The insulating film 655 may be a stack of any of the abovematerials.

Structural Example 2 of Transistor

In the transistor 730 in FIGS. 11A to 11D, the semiconductor 663 and theinsulating film 653 may be etched at the same time when the conductivefilm 673 is formed by etching. FIG. 13 shows an example of such a case.

FIG. 13 shows the case where the semiconductor 663 and the insulatingfilm 653 in FIG. 11B are provided only under the conductive film 673.

Structural Example 3 of Transistor

In the transistor 730 in FIGS. 11A to 11D, the conductive films 671 and672 may be in contact with side surfaces of the semiconductors 661 and662. FIG. 14 shows an example of such a case.

FIG. 14 shows the case where the conductive films 671 and 672 in FIG.11B are in contact with the side surfaces of the semiconductors 661 and662.

Structural Example 4 of Transistor

In the transistor 730 in FIGS. 11A to 11D, the conductive film 671 maybe a stack including a conductive film 671 a and a conductive film 671b. Furthermore, the conductive film 672 may be a stack including aconductive film 672 a and a conductive film 672 b. FIG. 15 shows anexample of such a case.

FIG. 15 shows the case where the conductive film 671 and the conductivefilm 672 in FIG. 11B are a stack including the conductive films 671 aand 671 b and a stack including the conductive films 672 a and 672 b,respectively.

The conductive films 671 b and 672 b may be formed using a transparentconductor, an oxide semiconductor, a nitride semiconductor, or anoxynitride semiconductor, for example. The conductive films 671 b and672 b may be formed using, for example, a film containing indium, tin,and oxygen, a film containing indium and zinc, a film containing indium,tungsten, and zinc, a film containing tin and zinc, a film containingzinc and gallium, a film containing zinc and aluminum, a film containingzinc and fluorine, a film containing zinc and boron, a film containingtin and antimony, a film containing tin and fluorine, a film containingtitanium and niobium, or the like. Alternatively, any of these films maycontain hydrogen, carbon, nitrogen, silicon, germanium, or argon.

The conductive films 671 b and 672 b may have a property of transmittingvisible light. Alternatively, the conductive films 671 b and 672 b mayhave a property of not transmitting visible light, ultraviolet light,infrared light, or an X-ray by reflecting or absorbing it. In somecases, such a property can suppress a change in electricalcharacteristics of the transistor due to stray light.

The conductive films 671 b and 672 b may preferably be formed using alayer that does not form a Schottky barrier with the semiconductor 662or the like. Accordingly, on-state characteristics of the transistor canbe improved.

The conductive films 671 a and 672 a may each have a single-layerstructure or a stacked-layer structure of a conductor containing, forexample, one or more of the following: boron, nitrogen, oxygen,fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese,cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum,ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy film ora compound film of the above element may be used, for example, and aconductor containing aluminum, a conductor containing copper andtitanium, a conductor containing copper and manganese, a conductorcontaining indium, tin, and oxygen, a conductor containing titanium andnitrogen, or the like may be used.

Note that the conductive films 671 b and 672 b preferably have higherresistance than the conductive films 671 a and 672 a according tocircumstances. The conductive films 671 b and 672 b preferably havelower resistance than the channel of the transistor according tocircumstances. For example, the conductive films 671 b and 672 b mayhave a resistivity of higher than or equal to 0.1 Ωcm and lower than orequal to 100 Ωcm, higher than or equal to 0.5 Ωcm and lower than orequal to 50 Ωcm, or higher than or equal to 1 Ωcm and lower than orequal to 10 Ωcm. The conductive films 671 b and 672 b having aresistivity within the above range can reduce electric fieldconcentration in a boundary portion between the channel and the drain.Therefore, a change in electrical characteristics of the transistor canbe suppressed. In addition, a punch-through current generated by anelectric field from the drain can be reduced. Thus, a transistor with asmall channel length can have favorable saturation characteristics. Notethat in a circuit configuration where the source and the drain do notinterchange, only one of the conductive films 671 b and 672 b (e.g., thefilm on the drain side) is preferably provided according tocircumstances.

<Crystal Structure of Oxide Semiconductor>

Next, the crystal structure of an oxide semiconductor that can be usedfor the semiconductor 662 will be described.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −30° and lessthan or equal to 30°. In addition, the term “perpendicular” indicatesthat the angle formed between two straight lines is greater than orequal to 80° and less than or equal to 100° and accordingly alsoincludes the case where the angle is greater than or equal to 85° andless than or equal to 95°. The term “substantially perpendicular”indicates that the angle formed between two straight lines is greaterthan or equal to 60° and less than or equal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

An oxide semiconductor film is classified into a non-single-crystaloxide semiconductor film and a single crystal oxide semiconductor film.Alternatively, an oxide semiconductor is classified into, for example, acrystalline oxide semiconductor and an amorphous oxide semiconductor.

Examples of a non-single-crystal oxide semiconductor include a c-axisaligned crystalline oxide semiconductor (CAAC-OS), a polycrystallineoxide semiconductor, a microcrystalline oxide semiconductor, and anamorphous oxide semiconductor. In addition, examples of a crystallineoxide semiconductor include a single crystal oxide semiconductor, aCAAC-OS, a polycrystalline oxide semiconductor, and a microcrystallineoxide semiconductor.

First, a CAAC-OS film will be described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image(also referred to as a high-resolution TEM image) of a bright-fieldimage and a diffraction pattern of the CAAC-OS film is observed.Consequently, a plurality of crystal parts are observed. However, in thehigh-resolution TEM image, a boundary between crystal parts, that is, agrain boundary is not clearly observed. Thus, in the CAAC-OS film, areduction in electron mobility due to the grain boundary is less likelyto occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a form that reflects unevenness of asurface over which the CAAC-OS film is formed (hereinafter, a surfaceover which the CAAC-OS film is formed is referred to as a formationsurface) or a top surface of the CAAC-OS film, and is arranged parallelto the formation surface or the top surface of the CAAC-OS film.

According to the high-resolution plan-view TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface, metal atoms are arranged in a triangular or hexagonalconfiguration in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Furthermore, a heavymetal such as iron or nickel, argon, carbon dioxide, or the like has alarge atomic radius (or molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film will be described.

A microcrystalline oxide semiconductor film has a region where a crystalpart is observed in a high resolution TEM image and a region where acrystal part is not clearly observed in a high resolution TEM image. Inmost cases, a crystal part in the microcrystalline oxide semiconductoris greater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In a high resolution TEMimage of the nc-OS film, for example, a grain boundary cannot be foundclearly in the nc-OS film sometimes.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. There is noregularity of crystal orientation between different crystal parts in thenc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Furthermore, a diffraction pattern like ahalo pattern appears in a selected-area electron diffraction pattern ofthe nc-OS film which is obtained by using an electron beam having aprobe diameter (e.g., larger than or equal to 50 nm) larger than thediameter of a crystal part. Meanwhile, spots are shown in a nanobeamelectron diffraction pattern of the nc-OS film obtained by using anelectron beam having a probe diameter close to, or smaller than thediameter of a crystal part. Furthermore, in a nanobeam electrondiffraction pattern of the nc-OS film, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS film, a plurality of spots isshown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Next, an amorphous oxide semiconductor film will be described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state as in quartz.

In the high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Furthermore, a halo pattern is shown but a spot is not shown in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may beseen. Furthermore, in the high-resolution TEM image, there are a regionwhere a crystal part is clearly observed and a region where a crystalpart is not observed. In the a-like OS film, crystallization by a slightamount of electron beam used for TEM observation occurs and growth ofthe crystal part is found sometimes. In contrast, crystallization by aslight amount of electron beam used for TEM observation is hardlyobserved in the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit cell of the InGaZnO₄ crystal has astructure in which nine layers, including three In—O layers and sixGa—Zn—O layers, are layered in the c-axis direction. Accordingly, thespacing between these adjacent layers is equivalent to the latticespacing on the (009) plane (also referred to as d value). The value iscalculated to be 0.29 nm from crystal structure analysis. Thus, focusingon lattice fringes in the high-resolution TEM image, each of latticefringes in which the lattice spacing therebetween is greater than orequal to 0.28 nm and less than or equal to 0.30 nm corresponds to thea-b plane of the InGaZnO₄ crystal.

The density of an oxide semiconductor film might vary depending on itsstructure. For example, if the composition of an oxide semiconductorfilm is determined, the structure of the oxide semiconductor film can beestimated from a comparison between the density of the oxidesemiconductor film and the density of a single crystal oxidesemiconductor film having the same composition as the oxidesemiconductor film. For example, the density of the a-like OS film ishigher than or equal to 78.6% and lower than 92.3% of the density of thesingle crystal oxide semiconductor having the same composition. Forexample, the density of each of the nc-OS film and the CAAC-OS film ishigher than or equal to 92.3% and lower than 100% of the density of thesingle crystal oxide semiconductor having the same composition. Notethat it is difficult to deposit an oxide semiconductor film whosedensity is lower than 78% of the density of the single crystal oxidesemiconductor film.

Specific examples of the above description will be given. For example,in the case of an oxide semiconductor film with an atomic ratio ofIn:Ga:Zn=1:1:1, the density of single-crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Thus, for example, in thecase of the oxide semiconductor film with an atomic ratio ofIn:Ga:Zn=1:1:1, the density of an a-like OS film is higher than or equalto 5.0 g/cm³ and lower than 5.9 g/cm³. In addition, for example, in thecase of the oxide semiconductor film with an atomic ratio ofIn:Ga:Zn=1:1:1, the density of an nc-OS film or a CAAC-OS film is higherthan or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that single crystals with the same composition do not exist in somecases. In such a case, by combining single crystals with differentcompositions at a given proportion, it is possible to calculate densitythat corresponds to the density of a single crystal with a desiredcomposition. The density of the single crystal with a desiredcomposition may be calculated using weighted average with respect to thecombination ratio of the single crystals with different compositions.Note that it is preferable to combine as few kinds of single crystals aspossible for density calculation.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, an a-like OSfilm, a microcrystalline oxide semiconductor film, and a CAAC-OS film,for example.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 4

In this embodiment, electronic devices of embodiments of the presentinvention will be described with reference to FIGS. 16A to 16F.

FIGS. 16A to 16F illustrate electronic devices. These electronic devicescan include a housing 5000, a display portion 5001, a speaker 5003, anLED lamp 5004, operation keys 5005 (including a power switch or anoperation switch), a connection terminal 5006, a sensor 5007 (a sensorhaving a function of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared ray), amicrophone 5008, and the like.

FIG. 16A illustrates a mobile computer, which can include a switch 5009,an infrared port 5010, and the like in addition to the above components.FIG. 16B illustrates a portable image reproducing device (e.g., a DVDplayer), which is provided with a memory medium and can include a seconddisplay portion 5002, a memory medium reading portion 5011, and the likein addition to the above components. FIG. 16C illustrates a goggle-typedisplay, which can include the second display portion 5002, a support5012, an earphone 5013, and the like in addition to the abovecomponents. FIG. 16D illustrates a portable game machine, which caninclude the memory medium reading portion 5011 and the like in additionto the above components. FIG. 16E illustrates a digital camera, whichhas a television reception function and can include an antenna 5014, ashutter button 5015, an image receiving portion 5016, and the like inaddition to the above components. FIG. 16F illustrates a portable gamemachine, which can include the second display portion 5002, the memorymedium reading portion 5011, and the like in addition to the abovecomponents.

The electronic devices illustrated in FIGS. 16A to 16F can have avariety of functions, such as a function of displaying a variety ofinformation (e.g., a still image, a moving image, and a text image) on adisplay portion, a touch panel function, a function of displaying acalendar, date, time, and the like, a function of controlling processingwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, and a function of reading a program or datastored in a recording medium and displaying the program or data on adisplay portion. Furthermore, the electronic device including aplurality of display portions can have a function of displaying imageinformation mainly on one display portion while displaying textinformation on another display portion, a function of displaying athree-dimensional image by displaying images where parallax is utilizedon a plurality of display portions, or the like. Furthermore, theelectronic device including an image receiving portion can have afunction of photographing a still image, a function of photographing amoving image, a function of automatically or manually correcting aphotographed image, a function of storing a photographed image in amemory medium (an external memory medium or a memory medium incorporatedin the camera), a function of displaying a photographed image on thedisplay portion, or the like. Note that functions that can be providedfor the electronic devices illustrated in FIGS. 16A to 16F are notlimited thereto, and the electronic devices can have a variety offunctions.

Each of the electronic devices described in this embodiment incorporatesa plurality of power storage elements and has a wireless receivingportion capable of wireless charging.

Usage examples of electronic devices are illustrated in FIGS. 17A and17B.

FIG. 17A shows an example where an information terminal is operated in amoving object such as a car.

The numeral 5103 indicates a steering wheel, which includes an antennainside. The antenna in the steering wheel 5103 can supply electric powerto an electronic device 5100. The electronic device 5100 has a pluralityof power storage elements, and at least one of the power storageelements is charged by wireless charging. The steering wheel 5103 may beprovided with a jig that can fix the electronic device 5100. If theelectronic device 5100 is fixed on the steering wheel 5103, the user canmake a phone call or a video-phone call without using his/her hands.Furthermore, through voice authentication with the use of a microphoneprovided in the electronic device 5100, the car can be driven by a voiceof the driver.

For example, by operating the electronic device 5100 while the car isparked, the positional information can be displayed on a display portion5102. Furthermore, information not displayed on a display portion 5101of the car, such as engine speed, steering wheel angle, temperature, andtire pressure may be displayed on the display portion 5102. The displayportion 5102 has a touch input function. Furthermore, one or morecameras to image the outside of the car can be used to display theoutside image on the display portion 5102. That is, the display portion5102 can be used as a back monitor, for example. Furthermore, forpreventing drowsy driving, the electronic device 5100 may operate asfollows, for example: while wirelessly receiving information such as thedriving speed from the car to monitor the driving speed, the electronicdevice 5100 images the driver at the time of driving and when a periodfor which the driver closes his/her eyes is long, it vibrates, beeps, orplays music (depending on the setting that can be selected by the driveras appropriate). Furthermore, by stopping imaging the driver while thecar is parked, power consumption can be reduced. In addition, the powerstorage elements of the electronic device 5100 may be wirelessly chargedwhile the car is parked.

The electronic device 5100 is expected to be used in a variety of waysin a moving object such as a car, as described above, and is desired toincorporate a number of sensors and a plurality of antennas that enablevarious functions thereof. Although a moving object such as a car has apower supply, the power supply is limited. In view of the electric powerto drive the moving object, it is preferable that the electric powerused for the electronic device 5100 be as low as possible. For anelectric vehicle, in particular, power consumed by the electronic device5100 may decrease the travel distance. Even if the electronic device5100 has a variety of functions, it is not often that all the functionsare used at a time, and only one or two functions are usually used asnecessary. In the case where the electronic device 5100 including aplurality of power storage elements, each of which is prepared for adifferent function, has a variety of functions, only the function to beused is turned on and electric power is supplied thereto from a powerstorage element corresponding to that function; whereby, powerconsumption can be reduced. Furthermore, power storage elementscorresponding to the functions not in use, among the plurality of powerstorage elements, can be wirelessly charged from an antenna provided inthe car.

FIG. 17B illustrates an example in which an information terminal isoperated in an airplane or the like. Since a period in which anindividual can use his/her own information terminal is limited in anairplane or the like, the airplane is desired to be equipped withinformation terminals that the passengers can use when the flight islong.

An electronic device 5200, having a display portion 5202 that displaysimages such as a movie, a game, and a commercial, is an informationterminal with which the current flying location and the remaining flighttime can be obtained in real time, owing to its communication function.The display portion 5202 has a touch input function.

The electronic device 5200 can be fit into a depressed portion in a seat5201, and an antenna installation portion 5203 is provided in a positionthat overlaps with the electronic device 5200, whereby the electronicdevice 5200 can be wirelessly charged while it is fit into the depressedportion. Furthermore, the electronic device 5200 can function as atelephone or communication tool when the user is sick and wants tocontact a flight attendant, for example. If the electronic device 5200has a translation function, the user can communicate with a flightattendant by using the display portion 5202 of the electronic device5200 even when the user and the flight attendant speak differentlanguages. Furthermore, passengers seated next to one another who speakdifferent languages can communicate by using the display portion 5202 ofthe electronic device 5200. In addition, the electronic device 5200 canfunction as a message board, displaying a message in English such as“please do not disturb” on the display portion 5202 while the user isasleep, for example.

The electronic device 5200 has a plurality of power storage elementseach of which is for a different function, and only the function to beused is turned on while the other functions not in use are in an offstate, whereby power consumption can be reduced. Furthermore, among theplurality of power storage elements, power storage elementscorresponding to the functions not in operation can be wirelesslycharged from the antenna installation portion 5203.

It is difficult to carry a dangerous object on an airplane. Theelectronic device 5200 having a plurality of small-sized power storageelements is highly safe, and even if one of the power storage elementsexplodes, the damage can be minimized because of its small size. Inaddition, even if one power storage element becomes unavailable becauseof failure, explosion, or breakage, some of the functions of theelectronic device 5200 can still be used by utilizing the other powerstorage elements.

The plurality of power storage elements of the electronic device 5200provided over the plurality of seats may be designed such that they canbe used in emergency when an airplane has an electrical problem. Sinceall the electronic devices 5200, each of which is provided for each ofthe plurality of seats, are the same products having the same design, asystem may be constructed such that the electronic devices 5200 can beconnected in series as an emergency power supply.

As the plurality of small-sized power storage elements of the electronicdevice 5200, one or more kinds selected from the following can be used:a lithium ion secondary battery such as a lithium polymer battery, alithium ion capacitor, an electric double-layer capacitor, and a redoxcapacitor.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 5

In this embodiment, an example of an artificial organ that is oneembodiment of the present invention will be described.

FIG. 18 is a cross-sectional schematic view of an example of apacemaker. A pacemaker body 5300 includes at least power storageelements 5301 a and 5301 b, a regulator, a control circuit, an antenna5304, a wire 5302 reaching a right atrium, and a wire 5303 reaching aright ventricle.

The pacemaker body 5300 is implanted in the body by surgery, and the twowires pass through a subclavian vein 5305 of the human body, with theend of one of them placed in the right ventricle and the end of theother of them placed in the right atrium.

The antenna 5304 can receive electric power, and the plurality of powerstorage elements 5301 a and 5301 b are charged with the electric power,which can reduce the frequency of replacing the pacemaker. Since thepacemaker body 5300 has a plurality of power storage elements, thesafety is high, and even when one of the power storage elements fails,the other can function. In this manner, the plurality of power storageelements function as auxiliary power supplies. Furthermore, if the powerstorage element to be provided in the pacemaker is further divided intoa plurality of thin power storage elements to be mounted on a printedboard where control circuits including a CPU and the like are provided,the pacemaker body 5300 can be smaller in size and thickness.

In addition to the antenna 5304 that can receive electric power, anantenna that can transmit a physiological signal may be provided for thepacemaker. For example, a system that monitors the cardiac activity,capable of monitoring physiological signals such as pulses, respiratoryrate, heart rate, and body temperature with an external monitoringdevice may be constructed.

If the pacemaker can be small in size and thickness according to thisembodiment, a protrusion generated in the portion where the pacemakerbody 5300 is implanted can be unnoticeably small.

Note that how the pacemaker is placed here is just an example, and itcan be changed in various ways depending on the heart disease.

Furthermore, this embodiment is not limited to the pacemaker. Anartificial ear is an artificial organ that is more widely used than thepacemaker. An artificial ear converts a sound into an electric signaland directly stimulates the auditory nerve with a stimulus device in thecochlea.

An artificial ear includes a first device implanted deep in the ear bysurgery and a second device that picks up sounds with a microphone andsends them to the implanted first device. The first device and thesecond device are not electrically connected to each other, andtransmission and reception between the two are conducted wirelessly. Thefirst device includes at least an antenna that receives an electricsignal converted from a sound and a wire that reaches the cochlea. Thesecond device includes at least a sound processing portion forconverting a sound into an electric signal and a transmitting circuitthat transmits the electric signal to the first device.

In this embodiment, a small-sized power storage element is provided ineach of the first device and the second device, whereby the artificialear can be reduced in size.

Since artificial ears are often implanted by surgery in childhood,reduction in size is desired.

If reduction in size of an artificial ear is achieved by thisembodiment, a protrusion generated in the portion where the artificialear is implanted can be unnoticeably small.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 6

In this embodiment, an example of a wearable electronic device that isone embodiment of the present invention will be described.

In the case where an electronic device with a complex shape ismanufactured, a plurality of small-sized power storage elements areplaced in predetermined places as appropriate, whereby the degree offreedom in design of the electronic device can be increased. As shown inFIG. 19A, an electronic device 5400 has a cylindrical form. In order forthe electronic device 5400 to be worn on the human body, a plurality ofpower storage elements rather than a single power storage element areappropriately placed, whereby a feeling of the weight can be reduced.Furthermore, if the device has a number of functions, consumption of apower storage element in a standby state increases; therefore, powerstorage elements for the respective functions are prepared. In the casewhere the electronic device 5400 having a plurality of power storageelements has a variety of functions, only the function to be used isturned on and electric power is supplied from the power storage elementcorresponding to the function, whereby power consumption can be reduced.

The electronic device 5400 is worn on the left upper arm, over a clothes5401, as shown in FIG. 19A. Examples of the clothes 5401 include clotheswith sleeves, such as a military uniform, an assault jacket, a suitjacket, a uniform, and space suits. There is no particular limitation onhow to put on the electronic device 5400, and examples of ways to put iton include sewing it on a portion of clothes that overlaps with theupper arm, attaching it with a Velcro fastener (registered trademark) orthe like provided on a portion of clothes that overlaps with the upperarm, fixing it with a band, a clasp, or the like, and binding aband-like leaf spring around an upper arm.

The electronic device 5400 has an antenna. A perspective view in whichthe electronic device 5400 is worn on the skin and wirelessly charged isshown in FIG. 19B. In FIG. 19B, the electronic device 5400 is worn on anupper arm 5402. A surface of the electronic device 5400 that is to be incontact with the skin is preferably formed using a skin-friendly film ora natural material such as leather, paper, and fabric. The numeral 5412indicates an electric power transmission device that can wirelesslycharge the electronic device 5400 with the use of a radio wave 5413.When provided with an antenna or a circuit that can transmit and receiveother data, the electronic device 5400 can transmit and receive otherdata as well as power. For example, the electronic device 5400 can alsobe used like a smartphone.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 7

In this embodiment, examples of electronic devices for which oneembodiment of the present invention can be used will be described withreference to FIGS. 20A to 20C.

FIG. 20A is a top view of a glasses-type device 5500, and FIG. 20B is aperspective view thereof.

The glasses-type device 5500 includes a portion that is positioned alongeach side of the head of the user when the device is worn (hereinafterreferred to as temples), and a plurality of power storage elements 5501are provided in each of the right and left temples.

In addition, the glasses-type device 5500 may include a terminal portion5504. The power storage elements 5501 can be charged through theterminal portion 5504. Furthermore, the power storage elements 5501 arepreferably electrically connected to each other. When the power storageelements 5501 are electrically connected to each other, all the powerstorage elements 5501 can be charged through the one terminal portion5504.

The glasses-type device 5500 may further include a display portion 5502.In addition, the glasses-type device 5500 may include a control portion5503. The control portion 5503 can control charge and discharge of thepower storage elements 5501 and can generate image data to be displayedon the display portion 5502. Moreover, if a chip having a wirelesscommunication function is included in the control portion 5503, data canbe transmitted to and received from the outside.

Alternatively, as illustrated in FIG. 20C, a glasses-type device 5510that is not provided with the display portion 5502 may be provided. Anexternal display portion 5512 may be attached to the glasses-type device5510. Thus, the distance between the eyes of the user and the displayportion 5512 can be easily adjusted.

Furthermore, between the glasses-type device 5510 and the externaldisplay portion 5512, wireless communication and wireless power feedingmay be performed.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 8

In this embodiment, an example of a semiconductor device that can beused for the device 10 described in Embodiment 1 will be described withreference to FIG. 21 and FIGS. 22A and 22B.

FIG. 21 is a cross-sectional view of a semiconductor device 1300 thatcan be used for the device 10.

The semiconductor device 1300 includes a substrate 700, a transistor300, a power storage element 740, an insulating film 741, an insulatingfilm 742, a wiring 743, and a wiring 708.

In the semiconductor device 1300, the transistor 300 and the powerstorage element 740 are provided over the same substrate.

For the details of the substrate 700, the power storage element 740, theinsulating films 741 and 742, and the wiring 708, the description of thesemiconductor device 1000 in shown in FIG. 2 is referred to.

The wiring 743 may have a function of a current collector of the powerstorage element 740, in addition to a function of a wiring. For thedetail of the wiring 743, the description of the current collector layer202 shown in FIGS. 5A to 5C is referred to.

Note that the regions without any reference numeral or hatching patternare regions made up of insulators in FIG. 21. The regions can be eachformed using an insulator containing at least one of aluminum oxide,aluminum nitride oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, tantalum oxide, and the like.Alternatively, in these regions, an organic resin such as a polyimideresin, a polyamide resin, an acrylic resin, a siloxane resin, an epoxyresin, or a phenol resin can be used.

Next, the detail of the transistor 300 will be described with referenceto FIGS. 22A and 22B.

FIGS. 22A and 22B are a top view and a cross-sectional view of thetransistor 300. FIG. 22A is the top view, and FIG. 22B corresponds tothe cross section in the dashed-dotted line A-B direction in FIG. 22A.In FIGS. 22A and 22B, some components are increased or reduced in size,or omitted for easy understanding. The dashed-dotted line A-B directionmay be referred to as a channel length direction.

The transistor 300 in FIG. 22B includes a conductive film 380functioning as a first gate, a conductive film 388 functioning as asecond gate, a semiconductor 382, conductive films 383 and 384functioning as a source and a drain, an insulating film 381, aninsulating film 385, an insulating film 386, and an insulating film 387.

The conductive film 380 is provided over an insulating surface. Theconductive film 380 and the semiconductor 382 overlap with each otherwith the insulating film 381 positioned therebetween. The conductivefilm 388 and the semiconductor 382 overlap with each other with theinsulating films 385, 386, and 387 positioned therebetween. Theconductive films 383 and 384 are connected to the semiconductor 382.

For the details of the conductive films 380 and 388, the descriptions ofthe conductive films 673 and 674 shown in FIGS. 11A to 11D are referredto.

The conductive films 380 and 388 may be supplied with differentpotentials, or may be supplied with the same potential at the same time.The provision of the conductive film 388 functioning as the second gateelectrode in the transistor 300 makes it possible to stabilize thethreshold voltage. The conductive film 388 may be omitted in some cases.

For the detail of the semiconductor 382, the description of thesemiconductor 662 shown in FIGS. 11B to 11D is referred to. Thesemiconductor 382 may be a single layer or a stack of a plurality ofsemiconductor layers.

For the details of the conductive films 383 and 384, the descriptions ofconductive films 671 and 672 shown in FIGS. 11A, 11B and 11D arereferred to.

For the detail of the insulating film 381, the description of theinsulating film 653 shown in FIGS. 11B to 11D is referred to.

Note that in FIG. 22B, the insulating films 385 to 387 are sequentiallystacked over the semiconductor 382 and the conductive films 383 and 384;however, the number of insulating films provided over the semiconductor382 and the conductive films 383 and 384 may be one or a plurality ofinsulating films may be stacked.

In the case where an oxide semiconductor is used for the semiconductor382, the insulating film 386 preferably contains oxygen at a proportionhigher than or equal to the stoichiometric composition and has afunction of supplying part of oxygen to the semiconductor 382 byheating. However, in the case where the provision of the insulating film386 directly on the semiconductor 382 causes damage to the semiconductor382 at the time of forming the insulating film 386, the insulating film385 is preferably provided between the semiconductor 382 and theinsulating film 386 as shown in FIG. 22B. The insulating film 385preferably causes little damage to the semiconductor 382 when theinsulating film 385 is formed compared to the case of the insulatingfilm 386 and has a function of passing oxygen. If the insulating film386 can be formed directly on the semiconductor 382 while suppressingdamage to the semiconductor 382, the insulating film 385 need notnecessarily be provided.

For the insulating films 385 and 386, a material containing siliconoxide or silicon oxynitride is preferably used, for example.Alternatively, a metal oxide such as aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, or hafnium oxynitride can be used.

The insulating film 387 preferably has an effect of blocking diffusionof oxygen, hydrogen, and water. Alternatively, the insulating film 387preferably has an effect of blocking diffusion of hydrogen and water.

As an insulating film has higher density and becomes denser or has afewer dangling bonds and becomes more chemically stable, the insulatingfilm has a higher blocking effect. An insulating film that has an effectof blocking diffusion of oxygen, hydrogen, and water can be formedusing, for example, aluminum oxide, aluminum oxynitride, gallium oxide,gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, orhafnium oxynitride. An insulating film that has an effect of blockingdiffusion of hydrogen and water can be formed using, for example,silicon nitride or silicon nitride oxide.

In the case where the insulating film 387 has an effect of blockingdiffusion of water, hydrogen, and the like, impurities such as water andhydrogen that exist in a resin in a panel or exist outside the panel canbe prevented from entering the semiconductor 382. In the case where anoxide semiconductor is used for the semiconductor 382, part of water orhydrogen entering the oxide semiconductor serves as an electron donor(donor). Thus, the use of the insulating film 387 having the blockingeffect can prevent a shift in threshold voltage of the transistor 300due to generation of donors.

In addition, in the case where an oxide semiconductor is used for thesemiconductor 382, when the insulating film 387 has an effect ofblocking diffusion of oxygen, diffusion of oxygen from the oxidesemiconductor to the outside can be prevented. Accordingly, oxygenvacancies in the oxide semiconductor that serve as donors are reduced,so that a shift in threshold voltage of the transistor 300 due togeneration of donors can be prevented.

Note that the transistor 300 may be used as the transistor 730 describedin Embodiment 1.

The semiconductor device 1300 can be used for the device 10. Forexample, fabricating the display portion 16 or the display drivercircuit 19 in the device 10 in FIG. 1 with the use of the transistor 300and fabricating the power storage element 17 with the use of the powerstorage element 740 can reduce the device 10 in size or thickness.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

This application is based on Japanese Patent Application serial no.2014-162455 filed with Japan Patent Office on Aug. 8, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor comprising a first gate electrode, a first source electrode,and a first drain electrode; a first insulating film over each of thefirst gate electrode, the first source electrode, and the first drainelectrode; an electric double-layer capacitor over the first insulatingfilm; a second insulating film over the electric double-layer capacitor;and a second transistor over the second insulating film; wherein thefirst transistor, the second transistor, and the electric double-layercapacitor are provided over one substrate, wherein the first transistorincludes a first semiconductor in a channel region, wherein the secondtransistor includes a second semiconductor in a channel region, whereina band gap of the second semiconductor is wider than a band gap of thefirst semiconductor, wherein one of a source and a drain of the secondtransistor is electrically connected to a current collector of theelectric double-layer capacitor, and wherein the electric double-layercapacitor includes a solid electrolyte.
 2. The semiconductor deviceaccording to claim 1, wherein the first semiconductor includes silicon,and wherein the second semiconductor includes an oxide semiconductor. 3.The semiconductor device according to claim 1, wherein the electricdouble-layer capacitor is capable of being charged wirelessly.
 4. Thesemiconductor device according to claim 1, wherein the substrate is asemiconductor substrate.
 5. The semiconductor device according to claim1, wherein the substrate is a flexible substrate.
 6. An electronicdevice comprising: the semiconductor device according to claim 1; and atleast one of a microphone, a speaker, a display portion, and anoperation key.
 7. A semiconductor device comprising: a first transistorcomprising a first gate electrode, a first source electrode, and a firstdrain electrode; a first insulating film over each of the first gateelectrode, the first source electrode, and the first drain electrode; anelectric double-layer capacitor over the first insulating film; a secondinsulating film over the electric double-layer capacitor; and a secondtransistor over the second insulating film; wherein the firsttransistor, the second transistor, and the electric double-layercapacitor are provided over one substrate, wherein the first transistorincludes a first semiconductor in a channel region, wherein the secondtransistor includes a second semiconductor in a channel region, whereinone of a source and a drain of the second transistor is electricallyconnected to a current collector of the electric double-layer capacitor,and wherein a band gap of the second semiconductor is wider than a bandgap of the first semiconductor.
 8. The semiconductor device according toclaim 7, wherein the first semiconductor includes silicon, and whereinthe second semiconductor includes an oxide semiconductor.
 9. Thesemiconductor device according to claim 7, wherein the electricdouble-layer capacitor is capable of being charged wirelessly.
 10. Thesemiconductor device according to claim 7, wherein the substrate is asemiconductor substrate.
 11. The semiconductor device according to claim7, wherein the substrate is a flexible substrate.
 12. An electronicdevice comprising: the semiconductor device according to claim 7; and atleast one of a microphone, a speaker, a display portion, and anoperation key.
 13. The semiconductor device according to claim 7,wherein one of the first source electrode and the first drain electrodeis electrically connected to the current collector of the electricdouble-layer capacitor.